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October 20th, 2013, 10:47 AM  #1 
Member Joined: Apr 2013 Posts: 52 Thanks: 0  Exercise with boxes and balls.
We have n boxes and m numbered (distinct) balls.With how many ways can we put the balls in the boxes so that no box contains more than one ball, if (a) n> = m, and (b) n <m Answer the same questions when the balls are identical (indistinguishable). I hope someone can help me to solve the exercise.Thanks in advance! 
October 20th, 2013, 05:49 PM  #2  
Math Team Joined: Dec 2006 From: Lexington, MA Posts: 3,267 Thanks: 408  Re: Exercise with boxes and balls. Hello, evinda! Part (b) is silly, isn't it? If there are fewer boxes than there are balls, at least one box will have more than one ball. Quote: Quote:
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October 21st, 2013, 01:26 AM  #3 
Senior Member Joined: Jun 2013 From: London, England Posts: 1,316 Thanks: 116  Re: Exercise with boxes and balls.
The important point to understand is that this problem is the same as choosing m boxes from a total of n boxes. Putting ball into a box equates to "choosing" the box. In the first case, you could imagine the balls being numbered 1m (but, in fact, the balls could have any distinct numbers or letters or be distinct colours  it doesn't matter). So, you are not only choosing m boxes, but they are ordered. So, this is exactly the same problem as choosing m boxes in order. In the second case, the balls are indistinguishable, so this is the same problem as choosing m boxes where the order does not matter. It's important to see the mathematical/probability equivalence of these problems. 
October 21st, 2013, 03:10 AM  #4 
Member Joined: Apr 2013 Posts: 52 Thanks: 0  Re: Exercise with boxes and balls.
Nice!!!I got it!!!Thank you very much guys!!! 

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