- #1
fleazo
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the problem:
In how many ways can we write the number 4 as the sum of 5 non-negative integers?I realize this is a generalized combinations problem. I can plug it in using a formula, but I want to understand the logic behind why the generalizaed combination formula works. More specifically, my book gives a description of the problem and I don't understand why it works at all. I have taken a screen cap of the solution that my book provides. Here it is
http://imgur.com/8BhxXPq
So I understand the concept of how they're explaining this - for example - one possible selection is I chose "box 1" 2 times, "box 2" 1 time, "box 3" 0 times, "box 4" 2 times, and "box 5" 0 times, this corresponds to : 2 + 1 + 2 = 5.
The confusion for me is, it seems like this approach would have a lot of repeats. For example - the selection of "box 1" 0 times, "box 2" 0 times, "box 3" 2 times, "box 4" 1 time, and "box 5" 2 times, would also produce : 2 + 1 + 2 = 5.I realize I'm missing something in the logic, but I'm not sure what.
C(r + n -1, r) (generalized combinations with repetitions allowed formula)
see the above section, this thread is about trying to understand a solution attempt in a textbook
In how many ways can we write the number 4 as the sum of 5 non-negative integers?I realize this is a generalized combinations problem. I can plug it in using a formula, but I want to understand the logic behind why the generalizaed combination formula works. More specifically, my book gives a description of the problem and I don't understand why it works at all. I have taken a screen cap of the solution that my book provides. Here it is
http://imgur.com/8BhxXPq
So I understand the concept of how they're explaining this - for example - one possible selection is I chose "box 1" 2 times, "box 2" 1 time, "box 3" 0 times, "box 4" 2 times, and "box 5" 0 times, this corresponds to : 2 + 1 + 2 = 5.
The confusion for me is, it seems like this approach would have a lot of repeats. For example - the selection of "box 1" 0 times, "box 2" 0 times, "box 3" 2 times, "box 4" 1 time, and "box 5" 2 times, would also produce : 2 + 1 + 2 = 5.I realize I'm missing something in the logic, but I'm not sure what.
Homework Equations
C(r + n -1, r) (generalized combinations with repetitions allowed formula)
The Attempt at a Solution
see the above section, this thread is about trying to understand a solution attempt in a textbook