# Probability question that I cant work out

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• Sep 26th 2011, 07:27 PM
tjcoll
Multiple attempt percentages
Hi all,
I answered questions from set A and was correct 64% of the time. Every time I was incorrect I had to answer a different set of questions (set B) and of these I was correct 66% of the time. When I was incorrect with a question from set B I had to answer a question from set C of which I was correct 70% of the time.

I was wondering what the probability of getting all three questions wrong is?
How many questions would I get correct from set A,B and C before I got all three wrong (on average)?
How many times would I expect to get all three answers wrong if I answered 1000 questions from set A?

Thanks so much for your help!
• Sep 26th 2011, 10:27 PM
tjcoll
Re: Multiple attempt percentages
So I've been going through all the treads trying to find something that may help me. Is it Binomial probability that I would need to use to find the answer?
Sorry for my ignorance... math was never my strong point!!
• Sep 26th 2011, 11:33 PM
CaptainBlack
Re: Multiple attempt percentages
Quote:

Originally Posted by tjcoll
Hi all,
I answered questions from set A and was correct 64% of the time. Every time I was incorrect I had to answer a different set of questions (set B) and of these I was correct 66% of the time. When I was incorrect with a question from set B I had to answer a question from set C of which I was correct 70% of the time.

I was wondering what the probability of getting all three questions wrong is?
How many questions would I get correct from set A,B and C before I got all three wrong (on average)?
How many times would I expect to get all three answers wrong if I answered 1000 questions from set A?

Thanks so much for your help!

The probability of getting all three wrong is (1-0.64)(1-0.66)(1-0.70)

CB
• Sep 27th 2011, 01:35 PM
tjcoll
Re: Multiple attempt percentages
Great, thanks!!