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Math Help - please help me working out the probablity for this?

  1. #1
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    please help me working out the probablity for this?

    The question is: A student is chosen at random, find the probability that they

    - do not attend the revision sessions but improve their grade. (which i have the answer as 0.04 and it is correct. )

    - improve their grade (answer = 0.85)

    Now, with that given information i need to do this:

    A student improves their grade, find the probability that they attended the revision sessions.
    I'm not sure how to do this, but the answer is 0.95 - please show me the steps for this so i can do the others.

    thank you
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  2. #2
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    Re: please help me working out the probablity for this?

    With just the information you give, there s no way to answer that question- and you were surely given additional information. How did you get the "0.04" and "0.85"?
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  3. #3
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    Re: please help me working out the probablity for this?

    oh gosh sorry - yes i thought i posted it but the second half didnt come up in my question.

    anyway a tree diagram was included, here is the link to it:
    Welcome to Flickr!

    thanks for your help
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  4. #4
    Super Member ILikeSerena's Avatar
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    Re: please help me working out the probablity for this?

    Quote Originally Posted by emily9843 View Post
    The question is: A student is chosen at random, find the probability that they

    - do not attend the revision sessions but improve their grade. (which i have the answer as 0.04 and it is correct. )

    - improve their grade (answer = 0.85)

    Now, with that given information i need to do this:

    A student improves their grade, find the probability that they attended the revision sessions.
    I'm not sure how to do this, but the answer is 0.95 - please show me the steps for this so i can do the others.

    thank you
    Quote Originally Posted by emily9843 View Post
    oh gosh sorry - yes i thought i posted it but the second half didnt come up in my question.

    anyway a tree diagram was included, here is the link to it:


    thanks for your help
    Hi emily9843!

    You are asked for a conditional probability.
    The probability on A given that B is true is:

    P(A | B) = \frac {P(A \wedge B)} {P(B)}

    In your case that is the chance that they attended, given that they improved, which is:

    \begin{aligned}P(attended|improved) &= \frac{P(attended \text{ and } improved)}{P(improved)} \\ &= \frac {0.9 \cdot 0.9} {0.9 \cdot 0.9 + 0.1 \cdot 0.4} = \frac{0.81}{0.85} = 0.95 \end{aligned}
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