Inv B 2013

[E=mc^2]

I think this has already been asked, but I still don't get it, I need a very detailed explanation.

Determine the number of non-negative integer solutions to the equation: p+q+r=24

I understood where the logic of the arranges comes from:

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Question is how do you get the possible arrangements.

Answer: 325

[Fredfredburger]

Well if you're at that point, it's a simple combination from then on.

You have 26 places and you're trying to choose where to put 2 bars. 26 choose 2.
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That's probably not detailed enough, sorry.

[E=mc^2]