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# Math Revolution DS Expert - Ask Me Anything about GMAT DS

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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8792
GMAT 1: 760 Q51 V42
GPA: 3.82

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10 Feb 2020, 00:52
[GMAT math practice question]

(Geometry) As the figure below shows, lines $$BD$$ and $$CE$$ are perpendicular to $$l$$. What is the length of $$DE$$?

Attachment:

2.10DS.png [ 5.69 KiB | Viewed 241 times ]

1) Triangle $$ABC$$ is a right isosceles triangle with $$AB = AC.$$

2) $$BD = 7$$ and $$CE = 15.$$
_________________
MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $79 for 1 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 8792 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 10 Feb 2020, 23:57 [GMAT math practice question] (Geometry) The figure shows parallelogram $$ABCD$$ and point $$E$$ on line $$AD$$. What is the ratio of $$△ABE : △DCE$$? Attachment: 2.11DS.png [ 15.24 KiB | Viewed 229 times ] 1) The area of $$ABCD$$ is $$50.$$ 2) $$AE : ED = 3 : 2.$$ _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$79 for 1 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"
Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8792
GMAT 1: 760 Q51 V42
GPA: 3.82

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12 Feb 2020, 00:21
MathRevolution wrote:
[GMAT math practice question]

(Geometry) As the figure below shows, lines $$BD$$ and $$CE$$ are perpendicular to $$l$$. What is the length of $$DE$$?

Attachment:
The attachment 2.10DS.png is no longer available

1) Triangle $$ABC$$ is a right isosceles triangle with $$AB = AC.$$

2) $$BD = 7$$ and $$CE = 15.$$

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since a triangle has $$3$$ variables, E is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)

Attachment:

2.10(A).png [ 8.97 KiB | Viewed 219 times ]

Since we have $$AB = CA, ∠ADB = ∠CEA = 90°$$ and $$∠BAD = 90° - ∠CAE = ∠ACE$$, triangles $$ABD$$ and $$CAE$$ are congruent.

Then we have $$BD = AE = 7$$ and $$AD = CE = 15.$$

Thus, $$DE = AD – AE = 15 – 7 = 8.$$

Since both conditions together yield a unique solution, they are sufficient.

In cases where 3 or more additional equations are required, such as for original conditions with “3 variables”, or “4 variables and 1 equation”, or “5 variables and 2 equations”, conditions 1) and 2) usually supply only one additional equation. Therefore, there is an 80% chance that E is the answer, a 15% chance that C is the answer, and a 5% chance that the answer is A, B, or D. Since E (i.e. conditions 1) & 2) are NOT sufficient when taken together) is most likely to be the answer, it is generally most efficient to begin by checking the sufficiency of conditions 1) and 2) when taken together. Obviously, there may be occasions on which the answer is A, B, C, or D.
_________________
MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $79 for 1 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 8792 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 12 Feb 2020, 00:22 [GMAT math practice question] (Geometry) Is $$OB = OC$$ in the figure below? Attachment: 2.12DS.png [ 4.22 KiB | Viewed 214 times ] 1) $$AD$$ is parallel to $$BC.$$ 2) $$ABCD$$ is an isosceles trapezoid. _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$79 for 1 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"
Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8792
GMAT 1: 760 Q51 V42
GPA: 3.82

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13 Feb 2020, 00:26
MathRevolution wrote:
[GMAT math practice question]

(Geometry) The figure shows parallelogram $$ABCD$$ and point $$E$$ on line $$AD$$. What is the ratio of $$△ABE : △DCE$$?

Attachment:
2.11DS.png

1) The area of $$ABCD$$ is $$50.$$

2) $$AE : ED = 3 : 2.$$

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.

Since triangles $$ABE$$ and $$DCE$$ have the same height, we have $$△ABE : △DCE = AE : ED.$$

Thus, condition 2) is sufficient.

Condition 1)
Since we don’t know $$AE : ED$$, condition 1) is not sufficient.

_________________
MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $79 for 1 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 8792 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 13 Feb 2020, 00:27 [GMAT math practice question] (Integers) What is the remainder of $$9^n-1$$ when it is divided by $$10$$? 1) $$n$$ is divisible by $$2$$. 2) $$n$$ is divisible by $$3$$. _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$79 for 1 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"
Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8792
GMAT 1: 760 Q51 V42
GPA: 3.82

### Show Tags

13 Feb 2020, 23:52
MathRevolution wrote:
[GMAT math practice question]

(Geometry) Is $$OB = OC$$ in the figure below?

Attachment:
The attachment 2.12DS.png is no longer available

1) $$AD$$ is parallel to $$BC.$$

2) $$ABCD$$ is an isosceles trapezoid.

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.

Since a quadrilateral has 5 variables, E is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)

Attachment:

2.12ds(a).png [ 6.37 KiB | Viewed 180 times ]

Since we have $$AB = DC, ∠ABC = ∠DCB$$, and $$BC$$ is a common side, triangles $$ABC$$ and $$DCB$$ are congruent.

Thus, $$∠OBC = ∠OCB$$ and triangle $$OBC$$ is an isosceles.

Then we have $$OB = OC.$$

Since both conditions together yield a unique solution, they are sufficient.

In cases where 3 or more additional equations are required, such as for original conditions with “3 variables”, or “4 variables and 1 equation”, or “5 variables and 2 equations”, conditions 1) and 2) usually supply only one additional equation. Therefore, there is an 80% chance that E is the answer, a 15% chance that C is the answer, and a 5% chance that the answer is A, B, or D. Since E (i.e. conditions 1) & 2) are NOT sufficient when taken together) is most likely to be the answer, it is generally most efficient to begin by checking the sufficiency of conditions 1) and 2) when taken together. Obviously, there may be occasions on which the answer is A, B, C, or D.
_________________
MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $79 for 1 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 8792 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 13 Feb 2020, 23:53 [GMAT math practice question] (Number Properties) For positive integers $$A$$ and $$B, G$$ is the greatest common divisor of $$A$$ and $$B$$, and $$L$$ is the least common multiple of $$A$$ and $$B$$. What is $$A + B$$? 1) $$L = 70$$ 2) $$\frac{G}{A} + \frac{G}{B} = \frac{7}{10}$$ _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$79 for 1 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"
Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8792
GMAT 1: 760 Q51 V42
GPA: 3.82

### Show Tags

16 Feb 2020, 17:53
MathRevolution wrote:
[GMAT math practice question]

(Integers) What is the remainder of $$9^n-1$$ when it is divided by $$10$$?

1) $$n$$ is divisible by $$2$$.

2) $$n$$ is divisible by $$3$$.

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.

$$9^1 = 9, 9^2 = 81 ~ 1, 9^3 = 729 ~ 9, 9^4 ~ 1, …$$

The odd number powers of $$9$$ have the units digit $$9$$ and the even number powers of $$9$$ have the units digits $$1.$$

Condition 1) tells us that $$n$$ is an even number and $$9^n – 1 ~ 1 – 1 = 0.$$

Thus condition 1) is sufficient.

Condition 2)
If $$n = 3$$, then we have $$9^3 – 1 ~ 9 – 1 = 8.$$ However, if $$n = 6$$, then we have $$9^6 – 1 ~ 1 – 1 = 0.$$ Therefore, condition 2) does not yield a unique solution.
Condition 2) is not sufficient.

If the original condition includes “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations,” etc., one more equation is required to answer the question. If each of conditions 1) and 2) provide an additional equation, there is a 59% chance that D is the answer, a 38% chance that A or B is the answer, and a 3% chance that the answer is C or E. Thus, answer D (conditions 1) and 2), when applied separately, are sufficient to answer the question) is most likely, but there may be cases where the answer is A, B, C, or E.
_________________
MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $79 for 1 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 8792 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 16 Feb 2020, 17:57 MathRevolution wrote: [GMAT math practice question] (Number Properties) For positive integers $$A$$ and $$B, G$$ is the greatest common divisor of $$A$$ and $$B$$, and $$L$$ is the least common multiple of $$A$$ and $$B$$. What is $$A + B$$? 1) $$L = 70$$ 2) $$\frac{G}{A} + \frac{G}{B} = \frac{7}{10}$$ => Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution. Visit https://www.mathrevolution.com/gmat/lesson for details. Since we have $$2$$ variables ($$A$$ and $$B$$) and $$0$$ equations, C is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first. Conditions 1) & 2) Assume $$A = aG$$ and $$B = bG$$ where $$a$$ and $$b$$ are relatively prime numbers. Since $$\frac{G}{A} + \frac{G}{B} = \frac{7}{10}$$ from condition 2), we have $$\frac{G}{A} + \frac{G}{B} = \frac{7}{10},$$ $$\frac{G}{aG} + \frac{G}{bG} = \frac{7}{10}$$ (substituting in $$A = aG$$ and $$B = bG$$), $$\frac{bG}{abG} + \frac{aG}{abG} = \frac{7}{10}$$ (getting a common denominator), $$\frac{(aG + bG)}{abG} = \frac{7}{10}$$ (adding the fractions), $$\frac{(A + B)}{L} = \frac{7}{10}$$ (substituting $$A = Ag$$, $$B = bG$$, and $$L = abG$$). Since $$L = 70$$ from condition 1), we have $$\frac{(A + B)}{L} = \frac{7}{10}, \frac{(A + B)}{70 }= \frac{7}{10}$$ or $$A + B = 49.$$ Since both conditions together yield a unique solution, they are sufficient. Since this question is a statistics question (one of the key question areas), CMT (Common Mistake Type) 4(A) of the VA (Variable Approach) method tells us that we should also check answers A and B. Condition 1) If $$A = 70$$ and $$B = 1$$, then we have $$A + B = 71.$$ If $$A = 70$$ and $$B = 2$$, then we have $$A + B = 72.$$ Since condition 1) does not yield a unique solution, it is not sufficient. Condition 2) Assume $$A = aG$$ and $$B = bG$$ where $$a$$ and $$b$$ are relatively prime numbers. $$\frac{G}{A} + \frac{G}{B} = \frac{1}{a} + \frac{1}{b} = \frac{7}{10}.$$ If $$a = 2, b = 5$$, and $$G = 1,$$ then we have $$A = aG = 2(1) = 2, B = bG = 5(1) = 5$$ and $$A + B = 2 + 5 = 7.$$ If $$a = 2, b = 5,$$ and $$G = 2,$$ then we have $$A = aG = 2(2) = 4, B = bG = 5(2) = 10$$ and $$A + B = 4 + 10 = 14.$$ Since condition 2) does not yield a unique solution, it is not sufficient. Therefore, C is the answer. Answer: C Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B, or D is the answer. This occurs in Common Mistake Types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D, or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E. _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$79 for 1 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"
Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8792
GMAT 1: 760 Q51 V42
GPA: 3.82

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17 Feb 2020, 00:31
[GMAT math practice question]

(Algebra) What is the value of $$x - y$$?

1)$$x + y = 9$$

2) $$xy = 2$$
_________________
MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $79 for 1 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 8792 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 18 Feb 2020, 01:57 [GMAT math practice question] (Equation) What is the value of $$abc$$? 1) $$a + \frac{4}{b} = 1$$ 2) $$b + \frac{1}{c} = 4$$ _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$79 for 1 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"
Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8792
GMAT 1: 760 Q51 V42
GPA: 3.82

### Show Tags

19 Feb 2020, 01:12
MathRevolution wrote:
[GMAT math practice question]

(Algebra) What is the value of $$x - y$$?

1)$$x + y = 9$$

2) $$xy = 2$$

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since we have $$2$$ variables ($$x$$ and $$y$$) and $$0$$ equations, C is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)

We have $$(x - y)^2 = (x + y)^2 - 4xy$$

Because
$$(x + y)^2 - 4xy$$

$$(x + y)(x + y) – 4xy$$

$$x^2+ xy + xy + y^2- 4xy$$

$$x^2 - 2xy + y^2$$

$$(x – y)(x – y)$$

$$(x – y)^2$$

Then, we substitute
$$(x + y)^2 - 4xy = 9^2 – 4*2 = 81 – 8 = 73.$$

Then, we have $$x – y = ±√73.$$

Since both conditions together do not yield a unique solution, they are not sufficient.

Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B, or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D, or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.
_________________
MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $79 for 1 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 8792 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 19 Feb 2020, 01:13 [GMAT math practice question] (Integer) What is the value of $$\sqrt{360 x}$$? 1) $$x$$ is a $$2$$ digit integer. 2) $$\sqrt{360 x}$$ is an integer. _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$79 for 1 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"
Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8792
GMAT 1: 760 Q51 V42
GPA: 3.82

### Show Tags

20 Feb 2020, 00:54
MathRevolution wrote:
[GMAT math practice question]

(Equation) What is the value of $$abc$$?

1) $$a + \frac{4}{b} = 1$$

2) $$b + \frac{1}{c} = 4$$

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.

Since we have $$3$$ variables ($$a, b$$, and $$c$$) and $$0$$ equations, E is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.
The question --------- is equivalent to ----------- for the following reason

Conditions 1) & 2)

When we multiply both sides of the equation $$a + \frac{4}{b} = 1$$ by $$b$$, we have $$ab + 4 = b.$$

When we multiply both sides of the equation $$b + \frac{1}{c} = 4$$ by $$c$$, we have $$bc + 1 = 4c$$ or $$bc = 4c – 1$$.

When we multiply both sides of the equation $$ab + 4 = b$$ by $$c$$, we have $$abc + 4c = bc.$$

When we replace $$bc$$ of the equation $$abc + 4c = bc$$ by $$4c – 1$$, we have $$abc + 4c = 4c – 1$$ or $$abc = -1$$.

Since both conditions together yield a unique solution, they are sufficient.

In cases where 3 or more additional equations are required, such as for original conditions with “3 variables”, or “4 variables and 1 equation”, or “5 variables and 2 equations”, conditions 1) and 2) usually supply only one additional equation. Therefore, there is an 80% chance that E is the answer, a 15% chance that C is the answer, and a 5% chance that the answer is A, B, or D. Since E (i.e. conditions 1) & 2) are NOT sufficient, when taken together) is most likely the answer, it is generally most efficient to begin by checking the sufficiency of conditions 1) and 2), when taken together. Obviously, there may be occasions on which the answer is A, B, C, or D.
_________________
MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $79 for 1 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 8792 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 20 Feb 2020, 00:55 [GMAT math practice question] (Absolute Values) What is the difference between the maximum and the minimum values of $$x$$? 1) $$x$$ satisfies $$2 < \sqrt{|x-2|} < 4.$$ 2) $$x$$ is an integer. _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$79 for 1 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"
Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8792
GMAT 1: 760 Q51 V42
GPA: 3.82

### Show Tags

21 Feb 2020, 00:44
MathRevolution wrote:
[GMAT math practice question]

(Integer) What is the value of $$\sqrt{360 x}$$?

1) $$x$$ is a $$2$$ digit integer.

2) $$\sqrt{360 x}$$ is an integer.

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since we have $$1$$ variable ($$x$$) and $$0$$ equations, D is most likely the answer. So, we should consider each condition on its own first.

Condition 1)

Since we don’t have a unique value from condition 1), it does not yield a unique solution, and it is not sufficient.

Condition 2)

$$10$$ and $$40$$ are possible values of $$x$$, since $$\sqrt{360*10} = \sqrt{3600} = 60$$ and $$\sqrt{360*40} = \sqrt{14400} = 120.$$

Since condition 2) does not yield a unique solution, it is not sufficient.

Conditions 1) & 2)

$$10$$ and $$40$$ are possible values of x satisfying both conditions as well.

Since both conditions together do not yield a unique solution, they are not sufficient.

If the original condition includes “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations” etc., one more equation is required to answer the question. If each of conditions 1) and 2) provide an additional equation, there is a 59% chance that D is the answer, a 38% chance that A or B is the answer, and a 3% chance that the answer is C or E. Thus, answer D (conditions 1) and 2), when applied separately, are sufficient to answer the question) is most likely, but there may be cases where the answer is A, B, C, or E.
_________________
MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $79 for 1 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 8792 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 21 Feb 2020, 00:46 [GMAT math practice question] (Algebra) What is $$a^3 - b^3$$? 1) $$\frac{1}{a}-\frac{1}{b}=2$$ 2) $$\frac{1}{a^2}+\frac{1}{b^2}=3$$ _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$79 for 1 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8792
GMAT 1: 760 Q51 V42
GPA: 3.82

### Show Tags

23 Feb 2020, 18:15
MathRevolution wrote:
[GMAT math practice question]

(Absolute Values) What is the difference between the maximum and the minimum values of $$x$$?

1) $$x$$ satisfies $$2 < \sqrt{|x-2|} < 4.$$

2) $$x$$ is an integer.

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since we have $$1$$ variable ($$x$$) and $$0$$ equations, D is most likely the answer. So, we should consider each condition on its own first.

Condition 1)

Since we have $$2 < \sqrt{|x-2|} < 4$$, we have $$4 < | x – 2 | < 16$$, by squaring everything.

Two Cases
First Case – positive value
$$(x – 2) = 4$$
$$x = 6$$

$$(x – 2) = 16$$
$$x = 18$$
Then we have $$6 < x < 18$$

Second Case – negative value
$$-(x – 2) = 4$$
$$-x + 2 = 4$$
$$-x = 2$$
$$x = -2$$

$$-(x - 2) = 16$$
$$-x + 2 = 16$$
$$-x = 14$$
$$x = -18$$
Then we have $$-14 < x < -2$$

It means we have $$-14 < x < -2$$ or $$6 < x < 18.$$
However, we don’t have either a maximum value of $$x$$ or a minimum value of $$x$$.

Since we can’t specify a unique solution, it is not sufficient.

Condition 2)

Since condition 2) does not yield a unique solution, it is not sufficient.

Conditions 1) & 2)
Using our solutions from earlier, namely $$-14 < x < -2$$ or $$6 < x < 18$$, we know that the possible values of $$x$$ satisfying both conditions are $$-13, -12, … , -3$$ and $$7, 8, … , 17.$$

The maximum value of $$x$$ is $$17$$ and the minimum value of $$x$$ is $$-13.$$

The difference between the maximum and the minimum value of $$x$$ is $$17 – (-13) = 30.$$

If the original condition includes “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations” etc., one more equation is required to answer the question. If each of conditions 1) and 2) provide an additional equation, there is a 59% chance that D is the answer, a 38% chance that A or B is the answer, and a 3% chance that the answer is C or E. Thus, answer D (conditions 1) and 2), when applied separately, are sufficient to answer the question) is most likely, but there may be cases where the answer is A, B, C, or E.
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"Only $79 for 1 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 8792 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 23 Feb 2020, 18:18 MathRevolution wrote: [GMAT math practice question] (Algebra) What is $$a^3 - b^3$$? 1) $$\frac{1}{a}-\frac{1}{b}=2$$ 2) $$\frac{1}{a^2}+\frac{1}{b^2}=3$$ => Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution. Visit https://www.mathrevolution.com/gmat/lesson for details. Since we have $$2$$ variables ($$a$$ and $$b$$) and $$0$$ equations, C is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first. Conditions 1) & 2) Since we have $$\frac{1}{a} – \frac{1}{b} = 2$$ $$\frac{b}{ab} – \frac{a}{ab} = 2$$ (common denominator) $$\frac{(b – a)}{ab} = 2$$ (subtracting fractions) $$a – b = -2ab$$ (multiplying both sides by $$ab$$) Since we have $$\frac{1}{a^2} + \frac{1}{b^2} = 3$$ $$\frac{b^2}{a^2b^2} + \frac{a^2}{a^2b^2} = 3$$ (common denominator) $$\frac{(b^2 + a^2)}{a^2b^2} = 3$$ (adding fractions) $$a^2 + b^2 = 3a^2b^2$$ (multiplying both sides by $$a^2b^2$$) Then we have $$a^2 + b^2 = (a-b)^2 + 2ab$$ $$a^2 + b^2 = (-2ab)^2 + 2ab$$ (substituting $$-2ab$$ in for $$a – b$$) $$a^2 + b^2 = 3a^2b^2$$ or $$a^2b^2 + 2ab = 0.$$ We have $$ab = -2$$ and $$a – b= -2ab = 4$$ from $$ab(ab+2)=0$$, since $$ab ≠ 0.$$ $$a^3-b^3 = (a-b)^3 + 3ab(a-b) = 4^3 + 3(-2)4 = 64 – 24 = 40.$$ Since both conditions together yield a unique solution, they are sufficient. Therefore, C is the answer. Answer: C Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B, or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D, or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E. _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$79 for 1 month Online Course"
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Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS   [#permalink] 23 Feb 2020, 18:18

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