Which expression below is a counterexample to the following statement?

Question

If a number is divisible by 3, then it is divisible by 6.
15 is divisible by 3, but is not divisible by 6.
42 is divisible by 3, so it is divisible by 6.
6 is divisible by 2 and 3, so it is divisible by 6
18 is divisible by 6, so it is divisible by 3.

Answer 1

15 is divisible by 3, but is not divisible by 6.