00:01
In this problem, we're given the probability that a occurs given that b has occurred is 0 .5, the probability that b occurs is 0 .9, and the probability that a occurs given that the complement of b has occurred is 0 .4.
00:14
Our job is to find the probability that b occurs given that a has occurred.
00:20
We will use bayes ' theorem, which states that the probability that a occurs given that b occurs is the probability of their intersection divided by the probability of the second event, in this case, the probability of b.
00:42
So we know two of the three parts of this formula in this problem.
00:48
We know the left -hand side is 0 .5.
00:53
We know that the probability that b occurs is 0 .9.
00:58
So what we can solve for is the probability that a intersect b occurs.
01:06
So we see that the probability that a intersect b occurs is simply the product of those two probabilities, or 0 .45.
01:21
All right, and we will need that answer in just a minute.
01:26
So what's the next step in this problem? we will apply the same formula, but the second application, we will use the version, the probability that a occurs given that b complement has occurred.
01:41
So let's apply this again.
01:43
Let's come down here and do that.
01:46
Here we have the probability that a occurs given that b complement occurs is the probability of their intersection divided by the probability that b complement occurs.
02:02
Now, we know two of these three.
02:05
We are given that the probability that a occurs given that b complement occurs is 0 .4.
02:12
We don't know the probability of the intersection.
02:16
If we look back at the problem at first glance, it may seem like we are not given this information.
02:22
But we are told that the probability that b does occur is 0 .9.
02:26
That means the probability that b does not occur, the probability of its complement is 1 minus that, or 0 .1...