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Advent calendar 2025

21 December

This puzzle was part of the 2025 Advent calendar.

All 2025 advent puzzles ~ More information

There are ten ways to make a list of four As and Bs that don't contain an even* number of consecutive As:

B,B,B,B
A,B,B,B
B,A,B,B
B,B,A,B
B,B,B,A

A,B,A,B
A,B,B,A
B,A,B,A
A,A,A,B
B,A,A,A

How many ways are there to make a list of eleven As and Bs that don't contain an even number of consecutive As?

* We don't count 0 consecutive As as being an even number of consecutive As.

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Tags: lists, sets, even numbers

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