but that's not how didildude explained, it's not just a matter of counting the number +1 or +2 or +3...

I think you have to add +1 to one of the digits, or to two of them, or to three of them..

example: he said 15 you can say: 2 (you just add 1+1), or you can say 6 (you just add 5+1), or 26 (1+1, 5+1) or 16 (1,5+1) or 25 (1+1,5).... that's all