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Math Help - Combinations

  1. #1
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    Combinations

    A student is writing a test with 4 true/false questions. The student wants to know the theoretical probability of passing the test by guessing. This means that 2 or more of the 4 questions are answered correctly by guessing. Determine this probability and show all your work.

    my work:

    .5 ^ 2

    +

    .5^3

    +
    .5^4
    Last edited by Jhevon; January 21st 2010 at 02:43 PM. Reason: restored deleted post.
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  2. #2
    Member
    Joined
    Mar 2009
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    Hello,

    This is a binomial probability question;
    eg. the probability of getting exactly 2 correct is
    4C2\cdot (0.5)^2 \cdot (0.5)^2=0.375
    pr of exactly 3 correct;
    4C3\cdot (0.5)^3 \cdot (0.5)^1=0.25
    and probaility of exactly 4;
    4C4 \cdot (0.5)^4 \cdot (0.5)^0=0.0625

    So the probability of passing \Pr(X\geq 2) where X is getting a correct answer is the sum of exactly 2, 3 and 4
    hope that helps...

    edit, oh... :P
    Last edited by Robb; January 21st 2010 at 02:33 PM. Reason: solved while posting help
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