In: Advanced Math
This is a combinatorics problem
Suppose we wish to find the number of integer solutions to the
equation below, where 3 ≤ x1 ≤ 9, 0 ≤ x2 ≤ 8, and
7 ≤ x3 ≤ 17.
x1 + x2 + x3 = r
Write a generating function for this problem, and use it to solve this problem for r = 20.