00:01
First, let's review the formula of a conditional probability.
00:06
The conditional probability of invent e given invent f equals the probability that the invent e and f happens at the same time divided by the probability of inventive.
00:24
So here, the inventor f is that we choose four balls from 15 balls.
00:33
And here, the sequence miters, hence we use permutation instead of combination.
00:42
So how many possibilities are there? there are 15 balls.
00:53
We choose the first one, there are 15 possibilities.
00:56
And then there are 14 balls left.
00:59
We choose the second one, there are 14 possibilities, and etc.
01:06
So what is the possibility that the fossil ball is white...