00:01
The question we have is as follows.
00:02
So if i question multiple choice quiz has five choices for each answer.
00:08
And so first of all, we want to find the probability of correctly guessing at random exactly one correct answer.
00:16
And so we also want to find the probability that we guess exactly two correct answers and exactly three correct answers.
00:24
So i want to draw your attention to certain conditions that we need.
00:30
The first one being that for each question, there are only two possible outcomes.
00:38
So if you pick a choice, it will either be correct or incorrect.
00:44
There is no other outcome for that.
00:49
So there are exactly two possible outcomes.
00:52
The second one is the probability of success is the same for each question.
00:57
So since you have five questions and only one of them can be correct, the probability of picking the correct choice is one by five.
01:12
Probability of picking the correct choice is one by five.
01:20
And so naturally the probability of picking an incorrect choice is four by five because you have four other choices.
01:30
Which are incorrect.
01:33
So the probability of picking an incorrect or a wrong choice is 4 by 5.
01:38
And so this is 1 minus p.
01:45
And so that's your second condition.
01:48
The third condition is that the events are independent.
01:52
So the answer or the correct choice to the first question does not influence the correct choice to the second question.
02:02
The answers to each question are independent.
02:08
So that's the third condition.
02:11
So all these conditions meet the criteria of a binomial distribution.
02:29
And so we want to find, first of all, the probability of one success.
02:40
So i'll just write the generic formula down first.
02:42
The probability of x successes is basically n choose x times the probability to the power x times 1 minus p to the par n minus x minus x so to explain all these uh variables first of all n is our number of observations and so in this case in case, n is going to be phi.
03:36
And p we know we calculated over here.
03:42
It's going to be 1 by 5.
03:46
Now x is going to change because we have three sub questions.
03:52
So first of all, we want to find the probability that we, when we guess randomly, that we get exactly one correct answer.
04:01
So in the first case, x is going to be 1.
04:05
Probability that x equals 1 so this is going to be phi choose 1 so if i were to expand this formula it would be phi factorial divide by 1 factorial times phi minus 1 factorial times probability which is 1 by 5 by 5 to the power x, which is just one, times 1 minus p, which is 1 minus 1 by 5, which is 4 by 5.
04:45
So notice this is the number of, or the probability that you pick a wrong choice for 5 times to the power n minus x...