00:01
Suppose we want to choose five letters without replacement from 15 distinct letters.
00:06
How many ways can this be done if order is relevant? well, when order is relevant, we're looking at a permutation.
00:13
So we want to take 15 items, five at a time.
00:17
So our formula for that is going to be n factorial over r factorial.
00:25
So we're going to say 15 factorial over 5 factorial.
00:31
So that's going to be 15 times 14 times 13 times 10 times 9 times 8, times 7, times 6 times 5 factorial.
00:41
And then that's going to be over 5 factorial.
00:44
So my 5 factorials will eliminate.
00:47
And then we can go and just multiply these values here.
00:50
So i'm going to say 15 times 14 times 13 times 10 times 9 times 8 times 7 and six...