Explanation

- Looking at the series, we can see that the numbers are increasing with some common differences between them.

- The pattern is as follows:

- 6 + 3 = 9

- 9 + 6 = 15

- 15 + 6 = 21

- 21 + 3 = 24

- 24 + 4 = 28

- 28 + 2 = 30

By observing the pattern, we can see that all numbers follow an increment by either 3 or 6, except for 24, which increases by 4 instead of 3 or 6.

Therefore, the odd man out in the series is 24.