Hi guerrero,

In these types of questions, you can use a math tactic called "prime factorization"

Here, we're told that z is a multiple of 9, so that means it has these factors:

z = (3)(3)(possibly other primes)

We're also told that w is a multiple of 4, so that means it has these factors:

w = (2)(2)(possibly other primes)

The question asks if zw is a multiple of 126? This is another way of asking if zw is made up of (2)(3)(3)(7)(possibly other primes)?

Since we have a 2 from the w and both 3s from the z, we really just need to know if we can find a 7 or not (this is what's referred to as "the question behind the question).

Fact 1 tells us that z is a multiple of 21. This means that, in addition to the (3)(3), z also has a 7
This is enough to confirm that zw is a multipole of 126.
Fact 1 is SUFFICIENT

Fact 2 tells us that w is a multiple of 25. This means that, in addition to the (2)(2), z also has (5)(5). MAYBE it has a 7, MAYBE it doesn't
Fact 2 is INSUFFICIENT

Final Answer: A

GMAT assassins aren't born, they're made,
Rich