1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Combinatorics question

  1. Apr 21, 2014 #1
    1. The problem statement, all variables and given/known data

    You have got 10 multiple choice questions with 3 possible choices per question. What is ur chance that you will have 5 answers correct?

    So the chance for a correct answer per question is 1/3 and for an uncorrect answer per question is 2/3.

    3. The attempt at a solution

    correct solution:

    10 nCr 5 * (1/3)^5 * (2/3)^5 = 896/6561

    (with 10 nCr 5 = combination with n= 10 and k = 5 )

    I don't know how to get to the solution.

    THank u
  2. jcsd
  3. Apr 21, 2014 #2


    User Avatar
    Homework Helper

    The expression is a simple application of Binomial probability. The final answer is just a simplification by cancellation of common factors.

    You first *choose* 5 questions that you get right. The other 5 are automatically decided by elimination. Then you figure out the probability of getting the 5 that you chose right and the other 5 wrong. Can you now see why the expression has that particular form?
  4. Apr 21, 2014 #3
    thank u
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted