t need help solving there problems. It's discrete Math.

7. List all 4-bit strings of weight 2. (recall that n-bit strings are strings of size n, with binary digits) 8. How many 7-bit string of weight 3 are there? 9. For a set of the following set A={I, J, K, L, M, N, P). how many subsets of size 3 are there? Explain how you obtained your