Answer:
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.
