Question

What's the next number?
1,5,9,13,17_?

Answer 1

1 + 4 = 5

5 + 4 = 9

9 + 4 = 13

13 + 4 = 17

17 + 4 = 21

Answer 2

see it is an Arithmetic Sequence.

and the common difference is +4

Answer 3

17, 21, 25...etc

Answer 4

1, 5, 9, 13, 17, 21, 25, 29, 33, 37, 41, 45 and so on.
Every number is 4 more than the previous number.