I don't know if there is any direct formula for such question, but it can be solved as follows:
S1 = { (1,1, 15), (1,2,14), (1,3,13),.., (1,15,1)} S2={(2,1,14), (2,2,13), ..(2,14,1)} S3={(3,1,13),.. (3,13,1)}, .., S15 ={(15,1,1,)}
so, we have 1 in units place (s1) 15 times, 2 -> 14 times , 3 -> 13 times and ... to 15 ->1 time.
so answer will be -> (15+14+13+...+1) for all possible places of digits to be covered.
so it is (15*16)/2 = 120.
So answer is 120.