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|m-n|=?

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Math Revolution GMAT Instructor
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|m-n|=?  [#permalink]

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New post 10 Jul 2018, 02:37
00:00
A
B
C
D
E

Difficulty:

  35% (medium)

Question Stats:

66% (00:41) correct 34% (00:36) wrong based on 38 sessions

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[GMAT math practice question]

|m-n|=?

1) m and n are integers
2) mn=13

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|m-n|=?  [#permalink]

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New post 10 Jul 2018, 03:20
MathRevolution wrote:
[GMAT math practice question]

|m-n|=?

1) m and n are integers
2) mn=13


1) m and n are integers

Clearly any integers could plugged in

m=2 & n=1........|m-n|=1

m=10 & n=1........|m-n|=9

m=2 & n=8........|m-n|=6

Insufficient

2) mn=13

m=13 & n=1........|m-n|=12

m=1 & n=13........|m-n|=12

m=26 & n=\(\frac{1}{2}\)........|m-n|=25.5

Insufficient

Combine 1 & 2

From 2: 13 is a prime number
From 1: m & n are integers

so either (m=13 & n=1) or (m=1 & n=13). In both cases.........|m-n|=12

Sufficient

Answer: C
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Re: |m-n|=?  [#permalink]

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New post 10 Jul 2018, 04:47
MathRevolution wrote:
[GMAT math practice question]

|m-n|=?

1) m and n are integers
2) mn=13


{m-n| = the distance between m and n.
Question stem, rephrased:
What is the distance between m and n?

Statement 1:
Clearly INSUFFICIENT.

Statement 2:
Case 1: m=13 and n=1, with the result that the distance between m and n is 12
Case 2: m=100 and n=13/100, with the result that the distance between m and n is about 100
Since the distance can be different values, INSUFFICIENT.

Statements combined:
Only four cases are possible:
m=13, n=1
m=-13, n=-1
m=1, n=13
m=-1, n=-13
In every case, the distance between m and n is 12.
SUFFICIENT.


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Re: |m-n|=?  [#permalink]

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New post 12 Jul 2018, 01:05
=>

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 m and n are integers, the pairs of solutions of the equation \(mn=13\) are
\(m = 1, n = 13; m = 13, n = 1; m = -1, n = -13\); and \(m = -13, n = -1.\)
If \(m = 1, n = 13,\) then \(| m – n | = | 1 – 13 | = 12.\)
If \(m = 13, n = 1\), then \(| m – n | = | 13 – 1 | = 12.\)
If \(m = -1, n = -13,\) then \(| m – n | = | -1 – (-13) | = 12.\)
If \(m = -13, n = -1,\) then \(| m – n | = | -13 – (-1) | = 12.\)

Since this question is an absolute value 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)

Since we don’t have enough information, condition 1) is not sufficient.

Condition 2)
If \(m = 1, n = 13,\) then \(| m – n | = | 1 – 13 | = 12.\)
If \(m = 2, n = \frac{13}{2}\), then \(| m – n | = | 2 – \frac{13}{2} | = \frac{9}{2}\).
Since we don’t have a unique solution, condition 2) 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.
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Re: |m-n|=? &nbs [#permalink] 12 Jul 2018, 01:05
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