16C15 = 16 when you get more familiar with this the answer will become obvious to you.
It is like saying how many ways can 15 obects be selected from 16 (order doesn't count)
and this is the same as saying how many ways can one object be omitted from the original 16. There is 16 objects so there are obviously 16 ways that one can be left out.
16Cr = 16C(16-r) it is always symmetrical like this.
There are different ways to enter this on the web2 calc
I would do it like this (if I had needed to)
nCr(16,15) = 16