I am working on this problem:

https://projecteuler.net/problem=15
I have a feeling that I need to be looking at combinatorics, specifically "choosing". I've also looked at Pascal's triangle but I still don't know how I would apply this to the problem.

Try to figure out a formula for getting "6" for the example they give you. Think about how many steps you'll have to take, no matter what way you go. Since you only go down or right, how many choices do you have for each step? If you write this like you do for "heads" and "tails" (but with D and R instead of H and T), what letter strings can you get? How many of those strings can you get? They probably explain it better

here.