Question 343404
21,(+1=) 22,(+2=) 24,(+1=) 25, (+4)= 29,(+1=) 30, (+6=) 36, (+1=)37 (+8=)...45

We notice that to go to the next number, we need to add 1, and to the next to add 2n.

So the next number is 37+8=45.

(It's not sure that this is the unique solution)