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Posted by **Nada** on Tuesday, May 8, 2012 at 9:14am.

can u explain it to me detailly.i have thought about this question for serveral hours but i still cant get the answer

- Maths(permutation) -
**MathMate**, Tuesday, May 8, 2012 at 10:34amThis is a tricky question.

The teacher prepares 5 books for 7 students, but did not say that all five books HAVE to be distributed. So my interpretation is that 0 to 5 books could have been distributed.

For the case of 0 book distributed, there is only one way: no one gets any book.

For 1 book distrbuted, there are 7 ways to distribute, one for each student.

For 2 books distributed, there are 7 ways for the first book, 6 ways for the second. The order of the books is not important because they are different. There are 7!/(7-2)! ways.

Continue until all 5 books are distributed for a total of

1+7+42+210+840+2520=3620 ways

- Maths(permutation) -
**Nada**, Tuesday, May 8, 2012 at 11:31ambut the correct ans is 2520,why?

- Maths(permutation) -
**MathMate**, Tuesday, May 8, 2012 at 1:05pmAs I indicated, this is a trick question.

I have assumed that not all five books need to be distributed because the question says the teacher "prepares" 5 books, and any student can take "one or no book". It did not say he "distributes" 5 books.

The answer assumes ALL five books are distributed, as you can verify with the last entry on my list, or 7!/2!

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