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Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 7612
GMAT 1: 760 Q51 V42 GPA: 3.82
If gcd(m,n) is the greatest common divisor of m and n, then n=?  [#permalink]

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Difficulty:   35% (medium)

Question Stats: 68% (01:22) correct 32% (01:13) wrong based on 43 sessions

### HideShow timer Statistics [GMAT math practice question]

If $$gcd(m,n)$$ is the greatest common divisor of $$m$$ and $$n$$, then $$n=?$$

1) $$gcd(m,n)=n$$
2) $$gcd(m,gcd(m,n))= 30$$

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MBA Section Director V
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Joined: 22 May 2017
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If gcd(m,n) is the greatest common divisor of m and n, then n=?  [#permalink]

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1
To find the value of n

Statement 1

$$gcd(m,n) = n$$

The values of m = 10, n = 2 satisfy the given statement.
The values of m = 10, n = 1 satisfy the given statement.

Hence statement 1 alone is not sufficient to find unique value of n

Statement 2

$$gcd(m,gcd(m,n)) = 30$$

The values of m = 30, n = 60 satisfy the given statement.
The values of m = 30, n =90 satisfy the given statement.

Hence statement 2 alone is not sufficient to find unique value of n

Statements 1 and 2 together

$$gcd(m,n) = n$$ and $$gcd(m,gcd(m,n))=30$$

Substituting the value of $$gcd(m,n)=n$$ in statement 2

=> $$gcd(m,n) = 30$$
=> $$n = 30$$ which gives a unique value for n

Hence option C
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Originally posted by workout on 11 Jul 2018, 02:11.
Last edited by workout on 11 Jul 2018, 04:45, edited 2 times in total.
Senior Manager  G
Joined: 04 Aug 2010
Posts: 435
Schools: Dartmouth College
If gcd(m,n) is the greatest common divisor of m and n, then n=?  [#permalink]

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MathRevolution wrote:
[GMAT math practice question]

If $$gcd(m,n)$$ is the greatest common divisor of $$m$$ and $$n$$, then $$n=?$$

1) $$gcd(m,n)=n$$
2) $$gcd(m,gcd(m,n))= 30$$

Statement 1:
Case 1: $$m=1$$ and $$n=1$$, with the result that $$gcd(m,n) = gcd(1,1) = 1$$
Case 2: $$m=2$$ and $$n=2$$, with the result that $$gcd(m,n) = gcd(2,2) = 2$$
Since $$n$$ can be different values, INSUFFICIENT.

Statement 2:
Case 1: $$m=30$$ and $$n=30$$, with the result that $$gcd(m,n) = gcd(30,30) = 30$$ and $$gcd(m,gcd(m,n)) = gcd(30,30) = 30$$
Case 2: $$m=30$$ and $$n=60$$, with the result that $$gcd(m,n) = gcd(30,60) = 30$$ and $$gcd(m,gcd(m,n)) = gcd(30,30) = 30$$
Since n can be different values, INSUFFICIENT.

Statements combined:
Substituting $$gcd(m,n)=n$$ into $$gcd(m,$$$$gcd(m,n)$$$$) =30$$, we get:
$$gcd(m,n)=30$$.
Substituting $$gcd(m,n)=30$$ into $$gcd(m,n)$$$$=n$$, we get:
$$30 = n$$.
SUFFICIENT.

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Originally posted by GMATGuruNY on 11 Jul 2018, 03:32.
Last edited by GMATGuruNY on 11 Jul 2018, 03:40, edited 1 time in total.
Senior Manager  G
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Re: If gcd(m,n) is the greatest common divisor of m and n, then n=?  [#permalink]

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1
workout wrote:
To find the value of n

Statement 1

$$gcd(m,n) = n$$

The values of m = 10, n = 0 satisfy the given statement.

Careful!
n=0 is not a valid case because 0 cannot be a divisor.
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Re: If gcd(m,n) is the greatest common divisor of m and n, then n=?  [#permalink]

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GMATGuruNY wrote:
workout wrote:
To find the value of n

Statement 1

$$gcd(m,n) = n$$

The values of m = 10, n = 0 satisfy the given statement.

Careful!
n=0 is not a valid case because 0 cannot be a divisor.

Ahh, yes, you are correct. Thanks for catching that.
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Re: If gcd(m,n) is the greatest common divisor of m and n, then n=?  [#permalink]

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=>
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)
In general, gcd(m,n) = gcd(m,gcd(m,n)).
By condition 1), gcd(m,gcd(m,n)) = n.
Therefore, by condition 2), n = gcd(m,gcd(m,n)) = 30.
Both conditions, together, are sufficient.

Since this question is an integer 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)
The condition that gcd(m,n) = n tells us only that m is a multiple of n.
Condition 1) is not sufficient.

Condition 2)

gcd(m,gcd(m,n)) = gcd(m,n) since m is a multiple of gcd(m,n).
Thus, gcd(m,n) = 30. But this does not allow us to determine a unique value of n.
Condition 2) is 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.
_________________ Re: If gcd(m,n) is the greatest common divisor of m and n, then n=?   [#permalink] 15 Jul 2018, 06:55
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