The digit are 1 to 9 . Each such number is strictly increasing sequence of digits, i.e., digits don't repeat and are in order. e.g., 12,135,1239 and soon. So, total such numbers in the number of increasing digit sequence of length 2 to 4 . We choose subsets of digits from 1 to 9 of lengths 2,3 and 4 in increasing order. ∴ 2-digits numbers =9C2=
