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Multiples and factors

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by ikaplan » Mon Jun 02, 2014 11:50 pm
If k is a positive integer and the greatest common divisor of k and 45 is 15, then the greatest common divisor of k and 900 may be any of the following EXCEPT

(A) 15
(B) 45
(C) 60
(D) 150
(E) 300
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Source: — Problem Solving |

by [email protected] » Tue Jun 03, 2014 12:31 am
Hi ikaplan,

This question is ultimately about "prime factorization" (the idea that a positive integer is either prime or can be "broken down" into the product of a bunch of primes).

We're told that the greatest common divisor of K and 45 is 15.

Since 45 = (3)(3)(5)
and 15 = (3)(5)
then K, at the minimum is (3)(5). It might have OTHER prime factors as well, BUT it can't have another 3. Here's why...

If K = (2)(3)(5) = 30, then the greatest common divisor of 30 and 45 is 15, so the above statement holds true.
If K = (3)(3)(5) = 45, then the greatest common divisor of 45 and 45 is 45, and the above statement doesn't make sense.

We're asked which of the following could be the greatest common divisor of K and 900 EXCEPT....

900 = (3)(3)(2)(2)(5)(5)
K = (3)(5)(possibly some other primes other than 3)

We can now use the answer choices to eliminate the possibilities:

A = 15 = (3)(5) is POSSIBLE
B = 45 = (3)(3)(5) is NOT POSSIBLE
C = 60 = (3)(5)(2)(2) is POSSIBLE
D = 150 = (3)(5)(2)(5) is POSSIBLE
E = 300 = (3)(5)(2)(2)(5) is POSSIBLE

Final Answer: B

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by theCodeToGMAT » Tue Jun 03, 2014 12:39 am
k = +ve


GCD = 15 = 5x3
that means 5x3 is common between k & 45

k = 5 x 3 x Y
45 = 5 x 3 x 3

THis implies that K cannot have another "3" or else GCD should have been 45


So, [spoiler]{B}[/spoiler]
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by GMATGuruNY » Tue Jun 03, 2014 2:16 am
ikaplan wrote:If k is a positive integer and the greatest common divisor of k and 45 is 15, then the greatest common divisor of k and 900 may be any of the following EXCEPT

(A) 15
(B) 45
(C) 60
(D) 150
(E) 300
Since the greatest integer that divides into both k and 45 is 15, k cannot be a multiple of 45.
To illustrate:
If k=90 -- a multiple of 45 -- then the GCF of k=90 and 45 would be 45, not 15.

Since k cannot be divisible by 45, the GCF of k and 900 cannot be divisible by 45.
Thus, the GCF of k and 900 cannot be answer choice B.

The correct answer is B.
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