Factorials


If you had 5 bricks and were taking them away at random, you could say:
The probability of selecting brick 1 is 1 ⁄ 5
The probability of selecting brick 2 is 1 ⁄ 4
The probability of selecting brick 3 is 1 ⁄ 3
The probability of selecting brick 4 is 1 ⁄ 2
The probability of selecting brick 5 is 1 ⁄ 1

So, to find the probability of selecting the bricks in that exact order is:
1 ⁄ 5 × 1 ⁄ 4 × 1 ⁄ 3 × 1 ⁄ 2 × 1 ⁄ 1

Thinking back to C1, we could shorten the denominator by using a factorial.
1 ⁄ 5!


We could put that into the calculator to produce a quick result.

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