In how many ways can 4 books be selected from a shelf of 6 books if the order in which the books are selected does not matter?
This problem is asking me to count the number of combinations of 4 books out of 6, without regard to order. I recall that the formula for the number of combinations of n objects taken r at a time is nCr = n!/(r!(n-r)!), where n! is the factorial of n, which means the product of all positive integers up to n. In this case, n = 6 and r = 4, so I can plug those values into the formula and simplify. Therefore, there are 15 ways to select 4 books from a shelf of 6 books, without regard to order. 