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January 30, 2015

January 30, 2015

Posted by **Martin** on Tuesday, November 1, 2011 at 11:54am.

- Discrete Math -
**MathMate**, Tuesday, November 1, 2011 at 4:41pmWithout any constraints, you have 30 to choose from for the first, 29 for the second, 28 for the third, and 27 for the fourth.

But there are 4! ways to choose the same 4 books, so using the multiplication principle, the number of ways to find 4 books is

30*29*28*27/4!

=30!/(4!(30-4)!)

=C(30,4)

The last line is a "function" called combination, or n choose r, where we choose r objects from n, and order of choice is not important.

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