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Math Help - last question

  1. #1
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    last question

    A student has to sell 2 books from a collection of 6 math, 7 science, and 4 economics books. How many choices are possible if
    a)both books are to be on the same subject
    b)the book are to be on different subject
    thanks
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  2. #2
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    Quote Originally Posted by mauro21pl View Post
    A student has to sell 2 books from a collection of 6 math, 7 science, and 4 economics books. How many choices are possible if
    a)both books are to be on the same subject
    The possibilities are:
    2 from 6 math
    2 from 7 science
    2 from 5 economics
    You shall find the number of combinations for each one add them them together for they are disjoint.
    {6\choose 2}+{7\choose 2}+{5\choose 2}
    b)the book are to be on different subject
    thanks
    The possibilities are:
    1 from 6 maths, 1 from 7 science
    1 from 6 maths, 1 from 4 economics
    1 from 7 science, 1 from 4 economics
    The add them up,
    {6\choose 1}\cdot {7\choose 1}+{6\choose 1}\cdot {4\choose 1}+{7\choose 1}\cdot {4\choose 1}
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