00:01
So we're taking the first six letters of the alphabet, a, b, c, d, e, f, and the first time we're going to make code words of four letters, but no letters repeated.
00:15
So no repeats.
00:16
Well, if you're choosing four items from a set of six and you're not allowing for repeats, and that means that the order matters, a, b, c, d is a different word than d, c, b, a, then we get permutations.
00:41
So we're going to do a permutation of six times six, choose four, so that is six factorial, divided by six minus four factorial, which is six times five times four times three, and that is equal to 360.
01:01
And this makes sense because the first letter we choose, we have six possibilities.
01:08
Then if we don't repeat that letter, there's only five letters left to choose from, so that gives us five options.
01:15
If we don't repeat the first two letters we've already used, then there's four options for that third letter...