15
Those are the factors of the number 60, in descending order. The next factor is 10.
The next number in the sequence 3-15-6-30-21 is -207.
There really is no way of telling, but my guess is 22.
10
The next number could be 26 The next number could be 12 - - - - - - - - - The next number that is in the sequence is 12.
15
21
Those are the factors of the number 60, in descending order. The next factor is 10.
17?
15
The next number in the sequence 3-15-6-30-21 is -207.
6 12 9 18 15 30 27 54 51
The next number in the sequence is 15. The pattern is as follows: from 1 to 6 is +5, from 6 to 9 +4, from 9 to 12 +3, from 12 to 14 +2, and then from 14 to the answer (15) +1.
It is 129.
10,20,30,20
There really is no way of telling, but my guess is 22.