GMATPrep multiples question

[allenkt]

Is integer n a multiple of 15?

a) n is a multiple of 20
b) n + 6 is a multiple of 3

[mschling52]

Is it C - Both combined sufficient?

(1) gives us that n is a multiple of 20, but 40 is not divisible by 15 while 60 is, so Not Sufficient.

(2) gives us that n+6 is a multiple of 3, hence n is also a multiple of 3. (since n+6 = 3k for some integer k, n = 3(k-2) for some integer k-2). However, 30 is divisible by 15, but 33 is not. Not Sufficient.

(1)&(2) imply that n is a multiple of 20 and of 3. Since 5 is a factor of 20, 5 and 3 will be factors of n, thus 15 is a factor of n. Sufficient.

[Stacey Koprince]

Yep, nice job!