Question

Find the number of ways of placing 4 balls in 5 boxes if the balls are distinguishable and the boxes are distinguishable.

Answer 1

Explanation:

let's start with combinations, where the order does not matter - the 4 basis are indistinguishable.

C(5, 4) = 5! / (4! × (5-4)!) = 5! / 4! = 5

that makes sense, of course, as we only need to trace the possibilities for the one empty box : 5 possibilities.

now, the order of the 4 balls matter.

the 4! term in the denominator was actually taking that away, so we need to remove it.

that turns the combinations into permutations, where the order matters.

P(5, 4) = 5! / (5 - 4)! = 5! = 5×4×3×2×1 = 120

4 distinguishable balls can be placed in 5 distinguishable boxes in 120 ways.