How many combinations are there when you can buy four books from a choice of 6 books>

Answer 1

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.