Verified answer

Lets split the series in two:

3,7,11,15

5,13,21,

The first progresses by 4 and the second progresses by 8.

So the next number would be 29.

3 then 2x3-1 =5

7 2x7-1 =13

11 2x11-1 =21

so 15 2x15-1= 29

29

thus first set of numbers 3,7,11,15 are increasing by 4 each time and second group 5,13,21,29 are increasing by 8

3,5,7,13,11,21,15 splits in two series:

3,7,11,15 : each step + 4

5,13,21 : each step + 8

next in the series is therefore 21 + 8 = 29

3,5,7 (+2 in series)

7,11,15(+4 in series)

5,13,21(+8 in series)

there fore 21+8=29

I don't believe there is a series here. Are you sure you typed it correctly?

Good job Rob S! Kudos to him.