Pascal's Triangle
Combinations are used in the binomial expansion theorem from algebra to give the coefficients of the expansion (a+b)^n. They also form a pattern known as Pascal's Triangle.
Each element in the table is the sum of the two elements directly above it. Each element is also a combination. The n value is the number of the row (start counting at zero) and the r value is the element in the row (start counting at zero). That would make the 20 in the next to last row C(6,3) -- it's in the row #6 (7th row) and position #3 (4th element).
