Combinatorics: the division rule
The division rule for counting things where we have a bunch of possible choices but want to consider some sets of choices as being the same, i.e. we don't care about certain differences between choices. Examples: putting people around circular tables, choosing which four soups to eat out of ten.

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4. Counting

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Combinatorics: the subtraction rule (PIE)

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Counting: Division Rule

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M131_COUNTING

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Terry Tao, Ph.D. Small and Large Gaps Between the Primes

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Math Olympiad Lecture 16 - Pigeonhole Principle

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The Oldest Unsolved Problem in Math

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Basics of Counting Part 1

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Counting The subtraction rule

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The Counting Principle || The Division Rule

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This pattern breaks, but for a good reason | Moser's circle problem

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