For questions about the study of finite or countable discrete structures, specifically how to count or enumerate elements in a set (perhaps of all possibilities) or any subset. It includes questions on permutations, combinations, bijective proofs, and generating functions.

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3
votes
2answers
71 views
+50

Number of ways through a modified grid

I was in math class listening to my teacher blather on about solids of revolution when my friend passed me a note with a puzzle on it. It looked something like this: How many different ways can you ...
8
votes
2answers
183 views
+50

How do I prove this combinatorial identity using inclusion and exclusion principle?

$$\binom{n}{m}-\binom{n}{m+1}+\binom{n}{m+2}-\cdots+(-1)^{n-m}\binom{n}{n}=\binom{n-1}{m-1}$$ Note that we can show this with out using inclusion and exclusion principle by using Pascal's Identity ...
7
votes
1answer
179 views
+200

Finding real money on a strange weighing device

You have 50 coins which each weigh either 20 grams or 10 grams. Each is labelled from 0 to 49 so you can tell the coins apart. You have one weighing device as well. At the first turn you can put as ...