How many positive numbers smaller than 10^n such that their digits are non-decreasing from left to right (e.g.?

answer
= [(n + 9)! / n!9!] - 1
= C(n+9,n) - 1

Solution write 9 for n times
Example
10^1; n=1 ; there are 9 positive numbers smaller than 10^1
10^4; n=4 ; there are 9999 positive numbers smaller than 10^4

Good luck!