00:01
Suppose that 60 % of students who take ap stats exam score 4 or 5, 25 % score 3, and the rest score 1 or 2.
00:06
Suppose further than 95 % of those scoring 4 or 5 receive college credit, 50 % of those scoring 3 receives such credit, and 4 % of those scoring 1 or 2 will receive the credit.
00:19
If a student who has chosen at a random from among those taking the exam receives college credit, what is the probability that she received a 3 on the exam? so we can draw a tree here, a three -part tree.
00:32
So we have 0 .6, 0 .25, and then 0 .15 will be, this will be 1 or 2.
00:43
This will be a 3 and this will be 4 or 5.
00:48
So here, it looks like we know that 0 .95 will get credit.
00:53
From here we know that 50 % will get credit and then here 4%, 0 .04.
00:58
So we multiply these probabilities across.
01:02
The probability of having a 4 of 5 and getting credit will be equal, we'll say probability, 4 ,5 and credit.
01:13
That'll equal, assuming these are independent, 0 .6 times 0 .95.
01:21
And then this is going to be probability of 3 in credits.
01:28
I'm abbreviating the credit to cr...