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# How many ordered pairs of positive integers (M,N) satisfy the equation

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Math Expert
Joined: 02 Sep 2009
Posts: 64068
How many ordered pairs of positive integers (M,N) satisfy the equation  [#permalink]

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22 Apr 2019, 00:46
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Difficulty:

55% (hard)

Question Stats:

54% (01:13) correct 46% (01:39) wrong based on 74 sessions

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How many ordered pairs of positive integers (M,N) satisfy the equation $$\frac {M}{6} = \frac{6}{N}$$ ?

(A) 6
(B) 7
(C) 8
(D) 9
(E) 10

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e-GMAT Representative
Joined: 04 Jan 2015
Posts: 3367
Re: How many ordered pairs of positive integers (M,N) satisfy the equation  [#permalink]

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22 Apr 2019, 02:30

Solution

Given:
• M and N are positive integers satisfying the equation M/6 = 6/N

To find:
• The number of ordered pairs satisfying the equation

Approach and Working:
• M/6 = 6/N
Or, MN = 36

As both M and N are positive integers, the number of ways we can write MN = 36 as follows:
• 1 x 36
• 2 x 18
• 3 x 12
• 4 x 9
• 6 x 6
• 9 x 4
• 12 x 3
• 18 x 2
• 36 x 1

Hence, the correct answer is option D.

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Joined: 12 Sep 2017
Posts: 306
Re: How many ordered pairs of positive integers (M,N) satisfy the equation  [#permalink]

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23 Apr 2019, 16:02
LCM of 36 = 9

Each factor will always be multiplied by another number.

Ex. 36 - a factor of 36.... 36*1 = 36

D
Target Test Prep Representative
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Re: How many ordered pairs of positive integers (M,N) satisfy the equation  [#permalink]

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26 Apr 2019, 14:33
2
Bunuel wrote:
How many ordered pairs of positive integers (M,N) satisfy the equation $$\frac {M}{6} = \frac{6}{N}$$ ?

(A) 6
(B) 7
(C) 8
(D) 9
(E) 10

Cross-multiplying, we see that MN = 36. Notice that if k is a positive factor of 36, then k x (36/k) = 36; therefore (k, 36/k) is one of the ordered pairs we are looking for. Since this is true for any positive factor of 36, the number of such ordered pairs is precisely the number of positive factors of 36.

Since 36 is a perfect square, we see that it has an odd number of factors, and thus the number of ordered pairs must also be odd, so the correct answer is either 7 or 9.

Since 36 = 2^2 x 3^2, it has (2 + 1)(2 + 1) = 9 factors and thus there are 9 ordered pairs (M, N) satisfying the equation.

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Re: How many ordered pairs of positive integers (M,N) satisfy the equation  [#permalink]

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31 Mar 2020, 04:42
Bunuel wrote:
How many ordered pairs of positive integers (M,N) satisfy the equation $$\frac {M}{6} = \frac{6}{N}$$ ?

(A) 6
(B) 7
(C) 8
(D) 9
(E) 10

Asked: How many ordered pairs of positive integers (M,N) satisfy the equation $$\frac {M}{6} = \frac{6}{N}$$ ?

M/6 = 6/N
MN = 36 ; N <>0

36 = 2^2 * 3^2

(M,N) = {(1,36),(2,18),(3,12),(4,9),(6,6),(9,4),(12,3),(18,2),(36,1)}

9 ordered pairs of M & N satisfy the equation

IMO D
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Kinshook Chaturvedi
Email: kinshook.chaturvedi@gmail.com
Re: How many ordered pairs of positive integers (M,N) satisfy the equation   [#permalink] 31 Mar 2020, 04:42