# Independent Events Question

• Oct 11th 2011, 06:55 PM
KelvinScale
Independent Events Question
Hello everyone,

I am having a bit of trouble with finding the answer to this question.
Students A, B, C each independently on a test. The probability of A getting the right answer is 0.9, for B it is 0.7 and for C it is 0.4.

If two of them get the right answer, what is the probability that C got their question wrong.

My logic behind this was finding AB(Not C) = 0.9*0.7*0.6, but apparently this is wrong. The correct answer was 0.7354

Any insight would be much appreciated. THanks!
• Oct 11th 2011, 09:28 PM
mr fantastic
Re: Independent Events Question
Quote:

Originally Posted by KelvinScale
Hello everyone,

I am having a bit of trouble with finding the answer to this question.
Students A, B, C each independently on a test. The probability of A getting the right answer is 0.9, for B it is 0.7 and for C it is 0.4.

If two of them get the right answer, what is the probability that C got their question wrong.

My logic behind this was finding AB(Not C) = 0.9*0.7*0.6, but apparently this is wrong. The correct answer was 0.7354

Any insight would be much appreciated. THanks!

It is conditional probability. You need to divide your answer by the probability that two of them got the right answer .....