Question

Any number that is divisible by 3 is also divisible by 6.

Find a counterexample to show that the conjecture is false.
  21
18
24
12

Answer 1

21/3=7
but
21/6=3.5
.......

Answer 2

21/3 = 7 21/6 = 3.5
18/3 = 6 18/6 = 3
24/3 = 8 24/6 = 4
12/3 = 4 12/6 = 2

Each can be divided to obtain an integer, except:

21/6 = 3.5

So, while 21 can be divided by 3, it cannot be divided by 6 (for an integer anyway)