Question

A time traveler can take 4 books with him, and he has 90 books to choose from. How many different ways can the books be selected?

Answer 1

The time traveler has 90 books to choose from, but he can only take 4 books

since we need to find the number of ways he can choose 4 books, we know that the order in which he takes the books does NOT matter

Hence, we will use a combination:

We know that the formula for combination is

`(n!)/(r!(n-r)!)`

in this case, n = 90, and r = 4. Thus, we get the following equation:

`(90!)/(4!(90-4)!)=(90!)/(4!(86)!)=2555190`

so that, the correct answer is:

`2555190`