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Math Help - exam answer sheets

  1. #1
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    exam answer sheets

    Shaniqua is taking a multiple choice exam and is going to guess all of her answers. There are 10 questions on the exam, each question with 3 possible choices and only one correct choice. What is the probability that she will make at least 60% on the exam just by guessing?

    Here is as far as I have gotten:

    there are 3^10 = 59049 possible ways to fill out the exam answer sheet. I know that I need to add the probabilities of getting 6/10 correct + prob7/10 + prob8/10 + prob9/10 + prob10/10

    i know that the probability of getting 10/10 by guessing is 1/59049, but I dont understand how to compute the probability of the others, can someone please point me in the right direction
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  2. #2
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    Quote Originally Posted by ihavvaquestion View Post
    Shaniqua is taking a multiple choice exam and is going to guess all of her answers. There are 10 questions on the exam, each question with 3 possible choices and only one correct choice. What is the probability that she will make at least 60% on the exam just by guessing?
    \sum\limits_{k = 6}^{10} {\binom{10}{k}\left( {\frac{1}<br />
{3}} \right)^k \left( {\frac{2}<br />
{3}} \right)^{10 - k} }
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  3. #3
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    I do not fully understand the notation...we are just beginning probability and are not allowed to use sums and limits in our answers.

    so from what i can tell for k=9, that would be getting 90% on the exam by guessing, would be computed as follows:

    10C9 * (1/3)^9 * (2/3)^1 =

    how would i compute the number of exam possibilities with 90% correct?
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  4. #4
    MHF Contributor matheagle's Avatar
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    this is just a binomial with n=10 and p=1/3

    You want  P(X\ge 6)= P(X=6)+  P(X=7)+  P(X=8)+  P(X=9)+ P(X=10)

    which is exactly the same as Plato's sum.
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