63
The next number in the sequence 3-15-6-30-21 is -207.
I'm guessing your sequence is 1, 3, 6, 10, 15, ... In which case it continues: 21, 28, 36, 45, 55, 66, ... (These are the triangular numbers.)
5
The next number in the sequence would be 25.
5
63
17?
15
The numbers are increasing by 1,4,9,16,25,36.... which are square numbers so the next increase is 49. Making the next number in the sequence 141
The next number in the sequence 3-15-6-30-21 is -207.
5+5 = 10 10+5 = 15 15-5 = 15 10+5 = 15
18
The next number could be 26 The next number could be 12 - - - - - - - - - The next number that is in the sequence is 12.
44. You're adding 11 then 9 each time !
.75
I'm guessing your sequence is 1, 3, 6, 10, 15, ... In which case it continues: 21, 28, 36, 45, 55, 66, ... (These are the triangular numbers.)