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

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Math Revolution GMAT Instructor
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06 Feb 2019, 07:10
MathRevolution wrote:
[GMAT math practice question]

(number properties) If $$x, y$$ are integers, is $$(x-y)(x+y)(x^2+y^2)$$ an odd number?

1) $$x$$ is an odd number
2) $$x-y$$ is an odd number

=>

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.

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

Since $$(x-y)(x+y)(x^2+y^2) = x^4-y^4$$, the question asks if $$x$$ and $$y$$ have different parities.

By Condition 2), $$x$$ and $$y$$ must have different parities since $$x – y$$ is an odd number.
Condition 2) is sufficient.

Condition 1)
Since we don’t know whether $$y$$ is even or odd, condition 1) is not sufficient.

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"Only $149 for 3 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: 6949 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 06 Feb 2019, 07:11 [GMAT math practice question] (inequality) If $$x, y, z$$ are positive integers, is $$xyz ≥ 64$$? $$1) xy ≥ yz ≥ zx ≥ 16$$ $$2) x+y+z=64$$ _________________ 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$149 for 3 month Online Course"
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06 Feb 2019, 08:04
fskilnik wrote:
Archit3110 wrote:
MathRevolution wrote:
[GMAT math practice question]

If $$a>b>c>d>0$$, is $$d<4$$?

$$1) \frac{1}{c} + \frac{1}{d} > \frac{1}{2}$$
$$2) (\frac{1}{a})+(\frac{1}{b})+(\frac{1}{c})+(\frac{1}{d})=1$$

#2
fraction of 1/a+1/b+1/c+1/d = 1
possible when all a=b=c=d are equal
so in sufficient to say that d<4
IMO A

Hi, Archit3110 !

Please check the parts in red.

Regards,
Fabio.

Fabio
understood thanks..
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Updated on: 08 Feb 2019, 04:14
MathRevolution wrote:
[GMAT math practice question]

(inequality) If $$x, y, z$$ are positive integers, is $$xyz ≥ 64$$?

$$1) xy ≥ yz ≥ zx ≥ 16$$
$$2) x+y+z=64$$

x,y,z are +ve integers

#1
$$1) xy ≥ yz ≥ zx ≥ 16$$

x=y=z; then only we can have $$1) xy ≥ yz ≥ zx ≥ 16$$
so x=y=z=4 atleast
so $$xyz ≥ 64$$; sufficient
#2

$$2) x+y+z=64$$
since x,y,z are + ve integers
so x= 62, y=z=1
so insufficient
IMO A
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Originally posted by Archit3110 on 06 Feb 2019, 08:12.
Last edited by Archit3110 on 08 Feb 2019, 04:14, edited 1 time in total.
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06 Feb 2019, 16:32
Archit3110 wrote:
Fabio
understood thanks..

I am glad to know it, Archit3110!

I found your post by coincidence... if you want to "mention" me, so that I know I have a post of yours like this one, do not forget to use my "nickname" fskilnik , please!

("Fabio" is related to someone with no posts, who left this forum in 2009... I have checked that one minute ago.)

Regards and success in your studies!
Fabio.
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Math Revolution GMAT Instructor
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07 Feb 2019, 17:24
MathRevolution wrote:
[GMAT math practice question]

If $$a>b>c>d>0$$, is $$d<4$$?

$$1) \frac{1}{c} + \frac{1}{d} > \frac{1}{2}$$
$$2) (\frac{1}{a})+(\frac{1}{b})+(\frac{1}{c})+(\frac{1}{d})=1$$

=>

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.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question.
The original condition $$a>b>c>d>0$$ is equivalent to $$0 < \frac{1}{a} < \frac{1}{b} < \frac{1}{c} < \frac{1}{d}$$. The question asks if $$d < 4$$. This is equivalent to asking if $$\frac{1}{d} > \frac{1}{4}$$.

By condition $$1, \frac{1}{d} > \frac{1}{4}$$ since $$\frac{1}{c} < \frac{1}{d}$$. So, $$d < 4$$. Condition 1) is sufficient.

Condition 2)
Since $$0 < \frac{1}{a} < \frac{1}{b} < \frac{1}{c} < \frac{1}{d}$$ and $$(\frac{1}{a})+(\frac{1}{b})+(\frac{1}{c})+(\frac{1}{d})=1$$, we have $$\frac{1}{a}<\frac{1}{d}$$, $$\frac{1}{b}<\frac{1}{d}, \frac{1}{c}<\frac{1}{d}$$ and $$\frac{1}{a} +\frac{1}{b} + \frac{1}{c} + \frac{1}{d} < \frac{1}{d} +\frac{1}{d} +\frac{1}{d} + \frac{1}{d} = \frac{4}{d}.$$
Therefore, $$1 < \frac{4}{d}$$ and $$d < 4$$.
Condition 2) is sufficient.

Note: This question is a CMT4(B) question: condition 1) is easy to work with and condition 2) is hard. For CMT4(B) questions, D is most likely to be the answer.
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"Only $149 for 3 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: 6949 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 07 Feb 2019, 17:26 [GMAT math practice question] (absolute value) $$3x+4y=?$$ $$1) 2|x|+3|y|=0$$ $$2) 3|x|+2|y|=0$$ _________________ 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$149 for 3 month Online Course"
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08 Feb 2019, 01:38
MathRevolution wrote:
[GMAT math practice question]

(inequality) If $$x, y, z$$ are positive integers, is $$xyz ≥ 64$$?

$$1) xy ≥ yz ≥ zx ≥ 16$$
$$2) x+y+z=64$$

=>

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.

Since we have $$3$$ variables ($$x, y, z$$) and $$0$$ equations, E is most likely to be the answer. So, we should consider each condition on its own first. As condition 1) includes 3 equations, we should consider it first.

Condition 1)
Since $$xy ≥ 16, yz ≥ 16$$, and $$zx ≥ 16$$, we have $$(xyz)^2 ≥ 16^3$$ or $$xyz ≥ 64$$.
Condition 1) is sufficient.

Condition 2)
If $$x = 20, y = 20$$ and $$z = 24$$, then $$xyz = 9600 ≥ 64$$ and the answer is ‘yes’.
If $$x = 1, y = 1$$ and $$z = 62$$, then $$xyz = 62 < 64$$ and the answer is ‘no’.
Since condition 2) does not yield a unique answer, condition 2) is not sufficient.

This is a CMT(Common Mistake Type) 4(A) question. If a question is from one of the key question areas and C should be the answer, CMT (Common Mistake Type) 4(A) of the VA (Variable Approach) method tells us that we should also check answers A and B.
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"Only $149 for 3 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: 6949 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 08 Feb 2019, 01:39 [GMAT math practice question] (function) Does $$f(x)=ax^4+bx^3+cx^2+dx+e$$ have ($$x-2$$) as a factor? $$1) f(2)=0$$ $$2) f(-2)=0$$ _________________ 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$149 for 3 month Online Course"
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08 Feb 2019, 03:57
MathRevolution wrote:
[GMAT math practice question]

(absolute value) $$3x+4y=?$$

$$1) 2|x|+3|y|=0$$
$$2) 3|x|+2|y|=0$$

IMO D

Modulus of any variable is a positive number.

if sum of 2 modulus variables is zero then it means the variable has a value zero

therefore 3x+4y = 0

Hence D
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08 Feb 2019, 04:21
MathRevolution wrote:
[GMAT math practice question]

(absolute value) $$3x+4y=?$$

$$1) 2|x|+3|y|=0$$
$$2) 3|x|+2|y|=0$$

#1 :
$$1) 2|x|+3|y|=0$$
whenx=y=0 sufficient
#2
again at x=y=0. sufficient
IMO D
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10 Feb 2019, 17:08
MathRevolution wrote:
[GMAT math practice question]

(absolute value) $$3x+4y=?$$

$$1) 2|x|+3|y|=0$$
$$2) 3|x|+2|y|=0$$

=>
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.

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

Condition 1)
Since $$|x| ≥ 0, |y| ≥ 0$$ and $$2|x| + 3|y| = 0,$$ we have $$x = y = 0.$$
Therefore, $$3x + 4y = 0.$$
Condition 1) is sufficient.

Condition 2)
Since $$|x| ≥ 0, |y| ≥ 0$$ and $$3|x| + 2|y| = 0$$, we have $$x = y = 0.$$
Therefore, $$3x + 4y = 0$$. Condition 2) is sufficient.

FYI: Tip 1) of the VA method states that D is most likely to be the answer if conditions 1) and 2) provide the same information.

_________________

MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
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"Only $149 for 3 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: 6949 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 10 Feb 2019, 17:09 MathRevolution wrote: [GMAT math practice question] (function) Does $$f(x)=ax^4+bx^3+cx^2+dx+e$$ have ($$x-2$$) as a factor? $$1) f(2)=0$$ $$2) f(-2)=0$$ => 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. The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. The statement that $$f(x)=ax^4+bx^3+cx^2+dx+e$$ has $$(x-2)$$ as a factor is equivalent to saying $$f(x) = (x-2)g(x)$$ for some function $$g(x)$$. This is equivalent to the requirement that $$f(2) = 0$$. Thus condition 1) is sufficient. Condition 2) If $$f(x) = (x-2)(x+2)(x-1)(x+1)$$, then $$f(x)$$ has $$x-2$$ as a factor, The answer is ‘yes’. If $$f(x) = x(x+2)(x+1)(x-1)$$, then $$f(x)$$ doesn’t have $$x-2$$ as a factor. The answer is ‘no’. Since condition 2) does not yield a unique answer, condition 2) is not sufficient. Therefore, the answer is A. Answer: A _________________ 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$149 for 3 month Online Course"
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11 Feb 2019, 01:28
[GMAT math practice question]

(geometry) What is the area of a right triangle?

1) The length of one side is $$3$$
2) The length of one side is $$4$$
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"Only $149 for 3 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: 6949 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 11 Feb 2019, 23:54 [GMAT math practice question] (inequality) Is $$3(a-b)>0$$? $$1) a^3>b^3$$ $$2) a>b$$ _________________ 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$149 for 3 month Online Course"
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13 Feb 2019, 00:49
MathRevolution wrote:
[GMAT math practice question]

(geometry) What is the area of a right triangle?

1) The length of one side is $$3$$
2) The length of one side is $$4$$

=>

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.

Since questions about triangles involve three variables, E is most likely to be the answer.

Conditions 1) & 2)
If the sides given are legs, then the area of the right triangle is $$(\frac{1}{2})*3*4 = 6.$$
If one leg has length $$3$$ and the hypotenuse has length $$4$$, then, by Pythagoras’ theorem, the length of the other leg is $$\sqrt{(4^2 – 3^2)} = √7$$, and the area of the triangle is $$(\frac{1}{2})*3*√7 =\frac{(3√7)}{2}.$$
Thus, both conditions together are not sufficient, since they do not yield a unique solution.

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.
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"Only $149 for 3 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: 6949 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 13 Feb 2019, 00:52 [GMAT math practice question] (algebra) $$x^3-y^3=?$$ $$1) x-y=0$$ $$2) |x|=|y|$$ _________________ 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$149 for 3 month Online Course"
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14 Feb 2019, 00:58
MathRevolution wrote:
[GMAT math practice question]

(inequality) Is $$3(a-b)>0$$?

$$1) a^3>b^3$$
$$2) a>b$$

=>

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.

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

Asking if $$3(a-b)>0$$ is equivalent to asking if $$a – b > 0$$, or, equivalently, if $$a > b.$$
Thus, condition 2) is sufficient.

Condition 1)
$$a^3>b^3$$
$$=> a^3-b^3 > 0$$
$$=> (a-b)(a^2+ab+b^2) > 0$$
$$=> a - b > 0$$ since $$a^2+ab+b^2 > 0$$
$$=> a > b$$
Thus, condition 1) is sufficient too.

FYI, Tip 1) of the VA method states that D is most likely to be the answer if conditions 1) and 2) provide the same information.
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"Only $149 for 3 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: 6949 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 14 Feb 2019, 01:00 [GMAT math practice question] (absolute value) Is $$|a|<1$$? $$1) a^2<1$$ $$2) \frac{1}{(1-a^2)}>0$$ _________________ 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$149 for 3 month Online Course"
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Math Revolution GMAT Instructor
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15 Feb 2019, 00:18
MathRevolution wrote:
[GMAT math practice question]

(algebra) $$x^3-y^3=?$$

$$1) x-y=0$$
$$2) |x|=|y|$$

=>

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.

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

$$x^3-y^3 = (x-y)(x^2+xy+y^2)$$

Condition 1)
If $$x – y = 0$$, then $$x^3-y^3 = (x-y)(x^2+xy+y^2) = 0.$$
Thus, condition 1) is sufficient.

Condition 2)
If $$x = 1$$ and $$y = 1$$, then $$x^3-y^3 = 1 – 1 = 0.$$
If $$x = 1$$ and $$y = -1$$, then $$x^3-y^3 = 1 –(-1) = 2.$$
Condition 2) is not sufficient since it does not yield a unique solution.

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Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS   [#permalink] 15 Feb 2019, 00:18

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