GMATPREP

If n is multiple of 15 then it can be
n = m(any number) x 15 = m x 5 x 3

1. n is a multiple of 20
that means n = a(any number) x 5 x 2 x 2
here if a is 3 , n will be multiple of 20.However, we are not sure.
So not sufficient.

2. n + 6 is a multiple of 3
This statement tells us that 3 is one of the factor of n. However, we don't know whether 5 is a factor of n.

Combining both the statements we know that 5(from 1) and 3 (from2) both are factors of n.Hence C is the correct ans.

Thanks. That helps a lot.

sochatte wrote:If n is multiple of 15 then it can be
n = m(any number) x 15 = m x 5 x 3

1. n is a multiple of 20
that means n = a(any number) x 5 x 2 x 2
here if a is 3 , n will be multiple of 20.However, we are not sure.
So not sufficient.

2. n + 6 is a multiple of 3
This statement tells us that 3 is one of the factor of n. However, we don't know whether 5 is a factor of n.

Combining both the statements we know that 5(from 1) and 3 (from2) both are factors of n.Hence C is the correct ans.

Just wanted to elaborate a little further that the trick really was to decipher statement (2) properly. (n+6) is a multiple of 3 can also be written in equation form as

(n+6) = 3(k), where k is a integer
=> n + 3(2) = 3(k)
=> n = 3(k) - 3(2)
=> n = 3 (k-2)
hence n is a multiple of 3... and then the rest follows...