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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- Jan 23rd 2007, 04:07 PM #1

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- Jan 23rd 2007, 04:15 PM #2

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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.

$\displaystyle {6\choose 2}+{7\choose 2}+{5\choose 2}$

b)the book are to be on different subject

thanks

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,

$\displaystyle {6\choose 1}\cdot {7\choose 1}+{6\choose 1}\cdot {4\choose 1}+{7\choose 1}\cdot {4\choose 1}$

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