How many combinations are there when you can buy four books from a choice of 6 books>
Combinations = n!/r!(n-r)!
where n is the number of things to choose from, and you choose r of them
(No repetition, order doesn't matter)
5 factorial = 5! = 5 * 4 * 3 * 2 * 1 = 120
I'll let you do the calculations.