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AK
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AK
Larry, Michael, and Doug have five donuts to share. If any one of the men can be given any whole number of donuts from 0 to 5, in how many different ways can the donuts be distributed?
(A) 21
(B) 42
(C) 120
(D) 504
(E) 5040

Please explain

------------------------------------
A..

5,0,0 -> 3
4,1,0 -> 3*2
3,2,0 -> 3*2
3,1,1 -> 3
2,2,1 -> 3
Total =21
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Andr359
Yep. A. You can see that a typical answer for this sort of questions is the sum of the series of integers from 1 to n+1 (n = number of items to distribute). In this case n = 5, thus the answer is 21. For n = 6 it´ll be 28. For n = 7, 36. And so on and so forth.


Excellent observation .....
Can you explain why it holds this pattern.. [1 ...+ (n +1)]?
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Because the formula (n+2)C2 is same as adding n + 1 numbers with first term as 1

(n+1){2*1 + (n+1-1)*1}/2 is the sum of arithmetic progression

Both give (n+2)(n+1)/2
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Andr359
Yep. A. You can see that a typical answer for this sort of questions is the sum of the series of integers from 1 to n+1 (n = number of items to distribute). In this case n = 5, thus the answer is 21. For n = 6 it´ll be 28. For n = 7, 36. And so on and so forth.

Excellent observation .....
Can you explain why it holds this pattern.. [1 ...+ (n +1)]?


Actually, for the particular case in which the number of people to whom distribute the items is 3, that formula holds. In general, for n people and I items to distribute, the formula is (I+n-1)Cr(n-1), or (I+n-1)! / (I! * (n-1)!). We verify that it works for the problem: I=5, n=3: (5+3-1)Cr(2) = 7Cr2 = 21.

When n=3: (I+3-1)Cr(3-1) = (I+2)Cr2 = (I+2)*(I+1)/2 = formula for the sum of integers from 1 to I+1.
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