00:01
In how many ways can three items be selected from a group of six? ok, so we are picking items without replacements.
00:09
We can't pick the same thing twice.
00:11
And the order doesn't matter.
00:14
We're just picking them out.
00:18
So this is a combinations question.
00:21
If the order did matter, it would be permutations.
00:23
If it were with replacements, we'd use the multiplication rule.
00:27
The formula for combinations n choose r is n factorial over r factorial n minus r factorial.
00:36
So we put 6 and 3 in here.
00:40
6 choose 3 is 20.
00:44
There are 20 ways this can happen.
00:47
And we have been asked to list them out.
00:49
So i'm going to try to do this in a very systematic way.
00:51
So we'll have, for example, abc is our first way.
00:54
Way.
00:54
A b d, a b e, a b f.
01:01
So that's our first four...