Find the odd number: 15, 21, 24, 28.

28 is the odd number.

15, 21 and 24 are all divisible by 3 (15 = 3 x 5, 21 = 3 x 7, 24 = 3 x 8).

Each of these three is a multiple of 3.

28 = 4 x 7 is not divisible by 3.

So the non-multiple of 3 stands apart.