Which of the following doesn't belong?

21-3-18-9-15

9 or 15

Step-by-step explanation:

If you choose 9 to be excluded from the list,

The list should be the following-

21-3-18-3-15

9 should become 3 to make it 15.

The sequence is created like this,

21 - 3 = 18

18 - 3 = 15

If you use 9, the result should be 18 - 9 = 9.

Therefore, if we choose 15 to be excluded from the sequence, the sequence should be-

21-3-18-9-9

So the answer is either 9 or 15.