The way to solve this kind of question!

ck1516

Math is my weakest link.

Is the integer n a multiple of 15?
1.n is a multiple of 20
2.n + 6 is a multiple of 3

Key is C.

Please tell me How do you solve it?
Thanks ahead!

redpot

multiple of 20 means it contains factor of 2; +6 is a multiple of 3 means it has 3 as its facotr.  Combined them together, you get it?

tongxun

doing such questions successfully, you must find the number' prime factors first.
if a number is a multiple of 15, it first contain factors 3 and 5 .
20=2*2*5, 60=3*2*5, so if n is a multiple of 15,n must contain factor 3. so we can know that 1) is not sufficient
n+6=3m, we only get n is a multiple of 3. but we do not know n have the factor 5, so 2) is not sufficient too.
1)+2)---we learn that n at least have two factors: 3 and 5, so n must be a multiple of 15

ck1516

This is it!

Perfect explainations!