2023 AMC 12A Problem 24
Problem 24 of 25HarderNumber TheoryCounting & Probability
Let be the number of sequences such that is a positive integer less than or equal to each is a subset of and is a subset of for each between and inclusive. For example, is one such sequence, with What is the remainder when is divided by
Answer choices
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Solution
For a fixed length each element of independently either never appears or first appears in one of giving choices. Hence there are chains of length
Summing, Modulo the terms reduce to which sum to
Thus, the correct answer is C.