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Thread: A probability question literally about homework

  1. #1

    A probability question literally about homework

    3. The homework diaries and completed homework of two Students A and B, are examined.

    There is a probability of 0.4 that Student A does not make a note in her diary homework set. She always does the homework if it is written in her diary, but never does the homework if it is not written in.

    There is a probability of 0.8 that Student B writes the homework given

    in her diary. When she does this, she will do the homework 90% of the time. If she has nothing written in her diary then she checks with a friend,

    who knows what the homework is 50% of the time; Student B always does the homework if she is told what it is by her friend.

    C) If one piece of homework was given to a student but wasn’t done, find the probability that it was given to Student A.
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  2. #2
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    Quote Originally Posted by megatkhairoll View Post
    3. The homework diaries and completed homework of two Students A and B, are examined.

    There is a probability of 0.4 that Student A does not make a note in her diary homework set. She always does the homework if it is written in her diary, but never does the homework if it is not written in.

    There is a probability of 0.8 that Student B writes the homework given

    in her diary. When she does this, she will do the homework 90% of the time. If she has nothing written in her diary then she checks with a friend,

    who knows what the homework is 50% of the time; Student B always does the homework if she is told what it is by her friend.

    C) If one piece of homework was given to a student but wasn’t done, find the probability that it was given to Student A.
    What are your thoughts?

    Please share your work with us ...even if you know it is wrong.

    If you are stuck at the beginning tell us and we'll start with the definitions.

    You need to read the rules of this forum. Please read the post titled "Read before Posting" at the following URL:

    http://www.freemathhelp.com/forum/announcement.php?f=33
    “... mathematics is only the art of saying the same thing in different words” - B. Russell

  3. #3
    Quote Originally Posted by Subhotosh Khan View Post
    What are your thoughts?


    Please share your work with us ...even if you know it is wrong.


    If you are stuck at the beginning tell us and we'll start with the definitions.


    You need to read the rules of this forum. Please read the post titled "Read before Posting" at the following URL:


    http://www.freemathhelp.com/forum/announcement.php?f=33
    Pardon my mistake there,
    Really sorry about that.


    Well, what I've did was actually making a ratio of ( probability of student A not doing the work) to ( the probability of students A and B not doing the work) because i believe i could find the chance of student A not doing the work.


    Values being

    0.4 / ( 0.4 + 0.18)

    Giving out 0.69.
    However I missed and the correct answer is 0.752.

    I couldn't upload my working due to a network errror
    Last edited by mmm4444bot; 02-07-2018 at 02:28 AM.

  4. #4
    Elite Member stapel's Avatar
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    Cool

    Quote Originally Posted by megatkhairoll View Post
    3. The homework diaries and completed homework of two Students A and B, are examined.

    There is a probability of 0.4 that Student A does not make a note in her diary homework set. She always does the homework if it is written in her diary, but never does the homework if it is not written in.

    There is a probability of 0.8 that Student B writes the homework given in her diary. When she does this, she will do the homework 90% of the time. If she has nothing written in her diary then she checks with a friend, who knows what the homework is 50% of the time; Student B always does the homework if she is told what it is by her friend.

    C) If one piece of homework was given to a student but wasn’t done, find the probability that it was given to Student A.
    Quote Originally Posted by megatkhairoll View Post
    Well, what I've [done] was actually making a ratio of (probability of student A not doing the work) to (the probability of students A and B not doing the work) because i believe i could find the chance of student A not doing the work.

    Values being 0.4 / ( 0.4 + 0.18)

    Giving out 0.69.

    However I missed and the correct answer is 0.752.

    I couldn't upload my working due to a network errror
    That's okay; you can type it out, just like you typed out the results, as displayed above.

    So please reply with your work and reasoning for the results you've posted, so that we can "see" what's going on. Thank you!

  5. #5
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    Quote Originally Posted by megatkhairoll View Post
    Pardon my mistake there,
    Really sorry about that.


    Well, what I've did was actually making a ratio of ( probability of student A not doing the work) to ( the probability of students A and B not doing the work) because i believe i could find the chance of student A not doing the work.


    Values being

    0.4 / ( 0.4 + 0.18)

    Giving out 0.69.
    However I missed and the correct answer is 0.752.

    I couldn't upload my working due to a network errror
    It sounds like, from your original post, that you only want help with part C? Because it also seems like that's what you are trying to figure out above.

    Did you get the correct answers for parts A and B already?

    For part C, I think where you are going wrong is that the probability of student A not doing the work is something different from what is being asked. What's being asked is, given that the homework being examined/checked was not completed, what's the probability that it was the one assigned to student A? In other words, the question has been flipped around. Instead of, given you assign the homework to student A, what's the probability it will get done, you're being asked the reverse: given the assigned homework wasn't done, what's the probability that you assigned it to student A?

    The keyword above is given. You're being asked to evaluate the probability under a certain given condition. This is a conditional probability. Do you know what these are? The conditional probability you want to compute is

    Probability it is student A given that the work was not done i.e. P(student A | not done)

    The probability you already know how to compute is

    P(not done | student A)

    So you have to figure out how to flip around the conditional probability. Can you think of any equations or theorems you've been given in class that would tell you how to flip around a conditional probability?

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