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Re: x and y are positive integers. If the greatest common divisor of 2x
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04 Apr 2017, 08:31

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GMATPrepNow wrote:

x and y are positive integers. If the greatest common divisor of 2x and 2y is 30, what is the greatest common divisor of x and 2y?

1) y is odd 2) x is odd

Target question:What is the greatest common divisor of x and 2y?

Given: the greatest common divisor of 2x and 2y is 30 30 = (2)(3)(5) This means that, if we examine the prime factorization of 2x and prime factorization of 2y, they will share exactly ONE 2, ONE 3, and ONE 5.

That is: 2x = (2)(3)(5)(?)(?)(?) 2y = (2)(3)(5)(?)(?)(?)

NOTE: Both prime factorizations might include other primes, BUT there is no additional overlap beyond the ONE 2, ONE 3, and ONE 5.

Notice that if we divide both sides of both prime factorizations by 2, we get: x = (3)(5)(?)(?)(?) y = (3)(5)(?)(?)(?) Since we already know that there is no additional overlap beyond the ONE 3, and ONE 5, we can conclude that the greatest common divisor (GCD) of x and y is 15.

Since we're trying to find the greatest common divisor of x and 2y, we should take a closer look at the prime factorizations of x and 2y: x = (3)(5)(?)(?)(?) 2y = (2)(3)(5)(?)(?)(?)

We already know that x and y have no additional overlap beyond the ONE 3, and ONE 5, the GCD of x and 2y will be EITHER 15 OR 30

If the prime factorization of x contains a 2, then x and 2y will share ONE 2, ONE 3, and ONE 5, which means the GCD of x and 2y will be 30

If the prime factorization of x does not contain a 2, then x and 2y will share ONE 3, and ONE 5, which means the GCD of x and 2y will be 15

So, it all comes down to whether or not the prime factorization of x contains a 2.

Statement 1: y is odd This information does not tell us whether or not the prime factorization of x contains a 2 There are several values of x and y that satisfy statement 1. Here are two: Case a: x = 15 and y = 15. This satisfies the given condition that the GCD of 2x and 2y is 30. In this case the GCD of x and 2y is 15 Case b: x = 30 and y = 15. This satisfies the given condition that the GCD of 2x and 2y is 30. In this case the GCD of x and 2y is 30 Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: x is odd If x is ODD, then we know that the prime factorization of x does not contain a 2, which means the GCD of x and 2y is 15 Since we can answer the target question with certainty, statement 2 is SUFFICIENT

Re: x and y are positive integers. If the greatest common divisor of 2x
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03 Apr 2017, 11:06

1

1

As per the information given in the question we can write 2x and 2y as below:

2 * x = 2 * 3 * 5 * a 2 * y = 2 * 3 * 5 * b

a and b above represent any possible factors of 2x and 2y, where a and b do not have any common prime factor

Statement 1) y is odd

If y is odd, we know that b is also odd. Now If a in 2*x is also odd, then the greatest common divisor of x and 2y is 3*5, but if a in 2*x is even, then the greatest common divisor of x and 2y is 2*3*5. As we have two possibilities, hence this statement is not sufficient.

Statement 2) x is odd

If x is odd, we know that a is also odd.

This means that x can be written as x = 3 * 5 * a (Where a is odd)

and 2 * y = 2 * 3 * 5 * b

As from the information given we already know a and b do not have any common prime factors, the greatest common divisor of x and 2y is 3*5 = 15

Re: x and y are positive integers. If the greatest common divisor of 2x
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25 Jun 2019, 07:29

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