00:01
So in this question, first of all, we're asked some hard questions where the probability of getting a correct answer is p.
00:16
And n is going to be the number of questions correctly answered without any mistakes.
00:34
And then we are, and then we are asked n easy questions with the probability of getting an answer is r, and the final score, why is the number of easy questions answered correctly? so first of all, let's find the pmf of n.
01:09
So the probability that n is equal to n is the probability of answering n questions correctly and then answering the following question wrong.
01:19
So it's the probability of answering n correctly and then answering the next one wrong, which is going to have probability 1 minus the probability of answering 1.
01:36
So this is going to be the probability of answering n correctly is just p to the n.
01:41
Then the probability of answering the next one incorrectly is 1 minus p.
01:49
So that's the pmf of n, and this is for n can be 0, 1, 2 and upwards.
02:04
So now we need to state the conditional distribution of y.
02:08
So the probability that y is equal to y given n is equal to n, well, well, y conditional on n, so actually that's write, y conditional on n equals n is going to be.
02:24
So we're asked n questions, and each of them we have a problem, the same probability r of answering.
02:31
Well, this is just a binomial experiment with n trials and an r chance of success on each trial.
02:36
So the probability that y is equal to y given n equals n, is n choose y times r to the y times one minus r to the n minus y.
02:53
Okay, so now part b we want to find the pmf of y.
02:58
Well, the pmf of y is the sum over possible values n, but n has got to be greater than or equal to y, because there's no way of answering y questions correctly if you have less than y questions to ask from.
03:20
So sum from n equals y up to infinity, the probability that y equals y given n equals n times the probability that n equals n.
03:34
So yeah, let's do that.
03:37
So with summing from n equals y to infinity, the probability y equals y given n equals n is n choose y.
03:45
So that's n factorial over y factorial, n minus y, factorial.
03:50
Times r to the y, 1 minus r to the n minus y, and then the probability that n equals n is just p to the n, 1 minus p.
04:02
So let's take out all the stuff that doesn't depend on n at all.
04:06
So we can pull out this r to the y and the 1 minus r to the minus y.
04:10
So we get r over 1 minus r to the y.
04:14
We can pull out the 1 minus p over the y factorial, so 1 minus p over y factorial.
04:21
And then we're summing from n equals y to infinity, n factorial over m minus y, factorial, and then 1 minus r to the power of m...