Just choose any 5 digits from 1 to 9 (omit 0 since your number has to have digits in increasing order, and we don't want 0 at the front)
So 9C5, and there's only 1 arrangement of them which is in ascending order. Total number of 5 digit numbers = 9*10^4 (since 0 can't be at the front)
Thus P =...