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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: 8763
GMAT 1: 760 Q51 V42
GPA: 3.82

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23 Dec 2018, 18:20
MathRevolution wrote:
[Math Revolution GMAT math practice question]

(integer) $$n$$ is a positive integer. Is $$\frac{n(n+1)(n+2)}{4}$$ an even integer?

1) $$n$$ is an even integer
2) $$1238 ≤ n ≤ 1240$$

=>

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 for $$\frac{n(n+1)(n+2)}{4}$$ to be an even integer is equivalent to asking for $$n(n+1)(n+2)$$ to be a multiple of $$8$$. If $$n$$ is an even integer, $$n$$ and $$n+2$$ are consecutive even integers and a product of two consecutive even integers is a multiple of $$8$$. Thus, condition 1) is sufficient.

Condition 2)
If $$n = 1238, n(n+1)(n+2)=1238*1239*1240$$ is a multiple of $$8$$ since $$1240$$ is a multiple of $$8$$.
If $$n = 1239, n(n+1)(n+2)=1239*1240*1241$$ is a multiple of $$8$$ since $$1240$$ is a multiple of $$8$$.
If $$n = 1240, n(n+1)(n+2)=1240*1241*1242$$ is a multiple of $$8$$ since $$1240$$ is a multiple of $$8$$.
Thus, condition 2) is sufficient.

Note: This question is a CMT4(B) question. Condition 1) is easy to understand and condition 2) is hard. When one condition is easy to understand, and the other is hard, D is most likely to be the answer.
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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: 8763 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 24 Dec 2018, 00:01 [Math Revolution GMAT math practice question] (number property) $$f(x)$$ is the greatest prime factor of $$x$$. If $$n$$ is a positive integer less than $$10$$, what is the value of $$n$$? $$1) f(n) = f(1000)$$ $$2) f(n) = 5$$ _________________ 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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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8763
GMAT 1: 760 Q51 V42
GPA: 3.82

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24 Dec 2018, 10:17
ShankSouljaBoi wrote:
MathRevolution wrote:
MathRevolution wrote:
[Math Revolution GMAT math practice question]

(number property) If $$n$$ is an integer greater than $$1$$, what is the value of $$n$$?

1) $$n$$ is a prime number
2) $$\frac{(n+2)}{n}$$ 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.

Since we have 1 variable (n) and 0 equations, D is most likely to be the answer. So, we should consider each of the conditions on their own first.

Condition 1)
Since there are many prime numbers, condition 1) is not sufficient.

Condition 2)
If $$n = 1$$, then $$\frac{(n+2)}{n} = 3$$ is an integer.
If $$n = 2,$$ then $$\frac{(n+2)}{n} = 2$$ is an integer.
Since we don’t have a unique solution, condition 2) is not sufficient.

Conditions 1) & 2)
If $$n = 2$$, then $$\frac{(n+2)}{n} = 2$$ is an integer.
If $$n = 3$$, then $$\frac{(n+2)}{n} = \frac{5}{2}$$ is not integer.
If $$n$$ is a prime number bigger than $$2$$, $$\frac{(n+2)}{n}$$ is not an integer.
Thus $$n = 2$$ is the unique solution and both conditions together are 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.

Please check the OA, Highlighted text is wrong. Stem alreay mention n as > 1.

Regards

You are right.
The question should be changed to the followings.

If $$n$$ is a positive integer, what is the value of $$n$$?

_________________
MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
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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: 8763 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 26 Dec 2018, 00:42 MathRevolution wrote: [Math Revolution GMAT math practice question] (number property) $$f(x)$$ is the greatest prime factor of $$x$$. If $$n$$ is a positive integer less than $$10$$, what is the value of $$n$$? $$1) f(n) = f(1000)$$ $$2) f(n) = 5$$ => 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 $$1$$ variable ($$n$$) and $$0$$ equations, D is most likely to be the answer. So, we should consider each condition on its own first. Condition 1) Since $$1000 = 2^3*5^3, f(1000) = 5.$$ So, $$f(n) = 5$$ and $$n = 5.$$ Thus, condition 1) is sufficient, since it gives a unique solution. Condition 2) Condition 2) is the same as condition 1). Thus, condition 2) is also 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. Therefore, D is the answer. Answer: D 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"
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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8763
GMAT 1: 760 Q51 V42
GPA: 3.82

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26 Dec 2018, 00:43
[Math Revolution GMAT math practice question]

(number properties) If $$m$$ and $$n$$ are positive integers, is $$m + n$$ an odd number?

1) $$\frac{m}{n}$$ is an even number
2) $$m$$ or $$n$$ is an even number
_________________
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" GMATH Teacher Status: GMATH founder Joined: 12 Oct 2010 Posts: 936 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 26 Dec 2018, 06:19 MathRevolution wrote: [Math Revolution GMAT math practice question] (number properties) If $$m$$ and $$n$$ are positive integers, is $$m + n$$ an odd number? 1) $$\frac{m}{n}$$ is an even number 2) $$m$$ or $$n$$ is an even number $$m,n\,\,\, \geqslant 1\,\,\,{\text{ints}}\,\,\,\,\left( * \right)$$ $$m + n\,\,\,\,\mathop = \limits^? \,\,{\text{odd}}\,\,\,\,\,\mathop \Leftrightarrow \limits^{\left( * \right)} \,\,\,\,\boxed{\,\,\,?\,\,\,:\,\,\,\left( {m\,\,{\text{odd}}\,,\,\,n\,\,{\text{even}}} \right)\,\,\,{\text{or}}\,\,\,{\text{vice - versa}\,\,}\,\,}$$ $$\left( 1 \right)\,\,\,\frac{m}{n} = {\text{even}}\,\,\,\,\left\{ \begin{gathered} \,{\text{Take}}\,\,\left( {m,n} \right) = \left( {2,1} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\text{YES}}} \right\rangle \,\, \hfill \\ \,{\text{Take}}\,\,\left( {m,n} \right) = \left( {4,2} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\text{NO}}} \right\rangle \,\, \hfill \\ \end{gathered} \right.$$ $$\left( 2 \right)\,\,\,m\,\,{\text{even}}\,\,\,{\text{or}}\,\,\,n\,\,{\text{even}}\,\,\,\,\left\{ \begin{gathered} \,\left( {\operatorname{Re} } \right){\text{Take}}\,\,\left( {m,n} \right) = \left( {2,1} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\text{YES}}} \right\rangle \,\, \hfill \\ \,\left( {\operatorname{Re} } \right){\text{Take}}\,\,\left( {m,n} \right) = \left( {4,2} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\text{NO}}} \right\rangle \,\, \hfill \\ \end{gathered} \right.\,\,\,\,\,\,\,\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,\,\,\,\,\,\,\,\left( {\text{E}} \right)$$ This solution follows the notations and rationale taught in the GMATH method. Regards, Fabio. P.S.: "A or B" means "only A", "only B" or BOTH. _________________ Fabio Skilnik :: GMATH method creator (Math for the GMAT) Our high-level "quant" preparation starts here: https://gmath.net GMATH Teacher Status: GMATH founder Joined: 12 Oct 2010 Posts: 936 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 26 Dec 2018, 06:21 MathRevolution wrote: [Math Revolution GMAT math practice question] (number properties) If $$n$$ is a positive integer, is $$\sqrt{n+1}$$ an even integer? 1) $$n$$ is the product of $$2$$ consecutive odd numbers 2) $$n$$ is an odd number Beautiful problem, Max. Congrats (and kudos)! $$n \geqslant 1\,\,\,\operatorname{int}$$ $$\sqrt {n + 1} \,\,\,\mathop = \limits^? \,\,{\text{even}}\,\,\,\,\,\,\,\mathop \Leftrightarrow \limits^{\left( * \right)} \,\,\,\,\,\,\boxed{\,\,n + 1\,\,\,\mathop = \limits^? \,\,\,{{\left( {{\text{even}}} \right)}^2}\,\,}$$ $$\left( 1 \right)\,\,\,n = \left( {2M - 1} \right)\left( {2M + 1} \right) = {\left( {2M} \right)^2} - {\left( 1 \right)^2}\,\,\,\,\,\left[ {M\,\,\operatorname{int} \,} \right]\,\,\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,\,\,\,n + 1 = {\left( {2M} \right)^2}\,\,\,,\,\,\,\,M\,\,\operatorname{int} \,\,\,\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,\,\,\,\left\langle {{\text{YES}}} \right\rangle \,$$ $$\left( 2 \right)\,\,\,\left\{ \matrix{ \,{\rm{Take}}\,\,n = 1\,\,\,\,\, \Rightarrow \,\,\,\,\,\left\langle {{\rm{NO}}} \right\rangle \,\, \hfill \cr \,{\rm{Take}}\,\,n = 3\,\,\,\,\, \Rightarrow \,\,\,\,\,\left\langle {{\rm{YES}}} \right\rangle \,\, \hfill \cr} \right.\,\,$$ This solution follows the notations and rationale taught in the GMATH method. Regards, Fabio. _________________ Fabio Skilnik :: GMATH method creator (Math for the GMAT) Our high-level "quant" preparation starts here: https://gmath.net GMAT Club Legend Joined: 18 Aug 2017 Posts: 6048 Location: India Concentration: Sustainability, Marketing GPA: 4 WE: Marketing (Energy and Utilities) Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 26 Dec 2018, 07:21 fskilnik wrote: MathRevolution wrote: [Math Revolution GMAT math practice question] (number properties) If $$m$$ and $$n$$ are positive integers, is $$m + n$$ an odd number? 1) $$\frac{m}{n}$$ is an even number 2) $$m$$ or $$n$$ is an even number $$m,n\,\,\, \geqslant 1\,\,\,{\text{ints}}\,\,\,\,\left( * \right)$$ $$m + n\,\,\,\,\mathop = \limits^? \,\,{\text{odd}}\,\,\,\,\,\mathop \Leftrightarrow \limits^{\left( * \right)} \,\,\,\,\boxed{\,\,\,?\,\,\,:\,\,\,\left( {m\,\,{\text{odd}}\,,\,\,n\,\,{\text{even}}} \right)\,\,\,{\text{or}}\,\,\,{\text{vice - versa}\,\,}\,\,}$$ $$\left( 1 \right)\,\,\,\frac{m}{n} = {\text{even}}\,\,\,\,\left\{ \begin{gathered} \,{\text{Take}}\,\,\left( {m,n} \right) = \left( {2,1} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\text{YES}}} \right\rangle \,\, \hfill \\ \,{\text{Take}}\,\,\left( {m,n} \right) = \left( {4,2} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\text{NO}}} \right\rangle \,\, \hfill \\ \end{gathered} \right.$$ $$\left( 2 \right)\,\,\,m\,\,{\text{even}}\,\,\,{\text{or}}\,\,\,n\,\,{\text{even}}\,\,\,\,\left\{ \begin{gathered} \,\left( {\operatorname{Re} } \right){\text{Take}}\,\,\left( {m,n} \right) = \left( {2,1} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\text{YES}}} \right\rangle \,\, \hfill \\ \,\left( {\operatorname{Re} } \right){\text{Take}}\,\,\left( {m,n} \right) = \left( {4,2} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\text{NO}}} \right\rangle \,\, \hfill \\ \end{gathered} \right.\,\,\,\,\,\,\,\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,\,\,\,\,\,\,\,\left( {\text{E}} \right)$$ This solution follows the notations and rationale taught in the GMATH method. Regards, Fabio. P.S.: "A or B" means "only A", "only B" or BOTH. fskilnik #2 says m or n even ; why have you taken both as even while proving statement as insufficient? GMATH Teacher Status: GMATH founder Joined: 12 Oct 2010 Posts: 936 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 26 Dec 2018, 08:35 Archit3110 wrote: #2 says m or n even ; why have you taken both as even while proving statement as insufficient? Hi Archit3110 , Thank you for your interest in my solution. As I explained in my post scriptum (PS), the word "OR" has not the everyday common use of "exclusive or". In other words, when it is given that "m is even or n is even", there are three possibilities available: (i) m is even and n is not even (ii) m is not even and n is even (iii) m is even and n is also even Regards and success in your studies, Fabio. _________________ Fabio Skilnik :: GMATH method creator (Math for the GMAT) Our high-level "quant" preparation starts here: https://gmath.net Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 8763 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 27 Dec 2018, 01:14 MathRevolution wrote: [Math Revolution GMAT math practice question] (number properties) If $$n$$ is a positive integer, is $$\sqrt{n+1}$$ an even integer? 1) $$n$$ is the product of $$2$$ consecutive odd numbers 2) $$n$$ 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. The question is equivalent to asking if $$\sqrt{n+1} = 2k$$ for some positive integer $$k$$. $$\sqrt{n+1} = 2k$$ $$=> n+1 = 4k^2$$ $$=> n = 4k^2-1$$ $$=> n = (2k-1)(2k+1)$$ $$n$$ is a product of two consecutive odd integers. Thus, condition 1) is sufficient. Condition 2) If $$n = 3$$, then $$\sqrt{3+1} = \sqrt{4}=2$$ and the answer is ‘yes’. If $$n = 1$$, then $$\sqrt{1+1} = 2$$ is not an integer and the answer is ‘no’. Condition 2) is not sufficient. Therefore, A is the answer. 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$79 for 1 month Online Course"
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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
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27 Dec 2018, 01:15
[Math Revolution GMAT math practice question]

(number properties) If $$m$$ and $$n$$ are positive integers, is $$3^{4m+2}+n$$ divisible by $$5$$?

$$1) m=3$$
$$2) n=1$$
_________________
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: 8763 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 28 Dec 2018, 02:17 MathRevolution wrote: [Math Revolution GMAT math practice question] (number properties) If $$m$$ and $$n$$ are positive integers, is $$m + n$$ an odd number? 1) $$\frac{m}{n}$$ is an even number 2) $$m$$ or $$n$$ is an even 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. Since we have $$2$$ variables ($$m$$ and $$n$$) and $$0$$ equations, C is most likely to be 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 $$\frac{m}{n}$$ is even, $$\frac{m}{n} = 2k$$ and $$m = (2k)n$$ for some positive integer $$k$$. If $$m = 2$$ and $$n = 1$$, then $$m + n = 3$$, is an odd integer and the answer is ‘yes’. If $$m = 4$$ and $$n = 2$$, then $$m + n = 6$$ is an even integer and the answer is ‘no’. Since we do not obtain a unique answer, conditions 1) & 2) are not sufficient when considered together. Therefore, E is the answer. Answer: E 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"
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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8763
GMAT 1: 760 Q51 V42
GPA: 3.82

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28 Dec 2018, 02:18
[Math Revolution GMAT math practice question]

(function) In the $$xy$$-coordinate plane, does $$y=a(x-h)^2+k$$ intersect the $$x$$-axis?

$$1) h=1$$
$$2) k=2$$
_________________
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: 8763 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 30 Dec 2018, 17:28 MathRevolution wrote: [Math Revolution GMAT math practice question] (number properties) If $$m$$ and $$n$$ are positive integers, is $$3^{4m+2}+n$$ divisible by $$5$$? $$1) m=3$$ $$2) n=1$$ => 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 units digits of $$3^k$$ have period $$4$$ as they form the cycle $$3 -> 9 -> 7 -> 1.$$ $$3^{4m+2}$$ has $$9$$ as its units digit if $$3^{4m+2}$$ has units digit $$9$$, regardless of the value of $$m$$. Thus, the divisibility of $$3^{4m+2}+n$$ by $$5$$ relies on the variable n only. Therefore, the correct answer is B. Answer: 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$79 for 1 month Online Course"
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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8763
GMAT 1: 760 Q51 V42
GPA: 3.82

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30 Dec 2018, 17:30
MathRevolution wrote:
[Math Revolution GMAT math practice question]

(function) In the $$xy$$-coordinate plane, does $$y=a(x-h)^2+k$$ intersect the $$x$$-axis?

$$1) h=1$$
$$2) k=2$$

=>

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.

Since we have $$3$$ variables ($$a, h$$ and $$k$$) and $$0$$ equations, E is most likely to be 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)
If $$a = 1$$, then the graph doesn’t intersect the $$x$$-axis shown as below.

Attachment:

1231.png [ 5.04 KiB | Viewed 292 times ]

If $$a = -1$$, then the graph intersects the $$x$$-axis shown as below.

Attachment:

12311.png [ 5.23 KiB | Viewed 292 times ]

Since neither condition gives us information about the value of $$a$$, conditions 1) & 2) are not sufficient, when considered together.

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 $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: 8763 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 31 Dec 2018, 01:23 [Math Revolution GMAT math practice question] (absolute value) Is $$\sqrt{(x+1)^2}=x+1$$ ? $$1) x(x-2) = 0$$ $$2) x(x+2) = 0$$ _________________ 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$79 for 1 month Online Course"
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Concentration: Sustainability, Marketing
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31 Dec 2018, 03:03
MathRevolution wrote:
[Math Revolution GMAT math practice question]

(absolute value) Is $$\sqrt{(x+1)^2}=x+1$$ ?

$$1) x(x-2) = 0$$
$$2) x(x+2) = 0$$

Given
$$\sqrt{(x+1)^2}=x+1$$
or say
$$\sqrt{(x+1)^2} = lx+1l lx+1l= x+1 only for values when x=0 or +/-1 so #1: [m]1) x(x-2) = 0$$
x=0 & x=+2

in sufficient
#2
$$2) x(x+2) = 0$$

x=0 & x=-2

in sufficient

from 1 & 2
x=0 sufficient

IMO C
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01 Jan 2019, 05:51
[Math Revolution GMAT math practice question]

(number properties) What is the value of the integer $$n$$?

1) $$n$$ is a prime factor of $$21$$
2) $$n$$ is a factor of $$49$$
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MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
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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: 8763 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS [#permalink] ### Show Tags 02 Jan 2019, 00:42 MathRevolution wrote: [Math Revolution GMAT math practice question] (absolute value) Is $$\sqrt{(x+1)^2}=x+1$$ ? $$1) x(x-2) = 0$$ $$2) x(x+2) = 0$$ => 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 question is equivalent to asking if $$x ≥ -1$$ as shown below: $$\sqrt{(x+1)^2}=x+1$$ $$=> |x+1| = x+1$$ $$=> x ≥ -1$$ Since we have $$1$$ variable ($$x$$) and $$0$$ equations, D is most likely to be the answer. So, we should consider each condition on its own first. Condition 1) $$x(x-2) = 0$$ $$=> x = 0$$ or $$x = 2$$ If $$x = 0$$, then $$x ≥ -1$$ and the answer is ‘yes’. If $$x = 2$$, then $$x ≥ -1$$ and the answer is ‘yes’. Since it gives a unique answer, condition 1) is sufficient. Condition 2) $$x(x+2) = 0$$ $$=> x = 0$$ or $$x = -2$$ If $$x = 0$$, then $$x ≥ -1$$ and the answer is ‘yes’. If $$x = -2$$, then $$x < -1$$ and the answer is ‘no’. Since it does not give a unique answer, condition 2) is not sufficient. Therefore, A is the answer. Answer: A 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: 8763
GMAT 1: 760 Q51 V42
GPA: 3.82

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02 Jan 2019, 00:45
[Math Revolution GMAT math practice question]

(number properties) Can $$n$$ be expressed as the difference of $$2$$ prime numbers?

$$1) (n-17)(n-21) = 0$$
$$2) (n-15)(n-17)=0$$
_________________
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 \$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"
Re: Math Revolution DS Expert - Ask Me Anything about GMAT DS   [#permalink] 02 Jan 2019, 00:45

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