There are $12$ chairs arranged in a circle, numbered from $ 1$ to $ 12$. How many ways are there to select some of the chairs in such a way that our selection includes $3$ consecutive chairs somewhere?
Problem
Source: 2019 Dürer Math Competition Finals Day2 E+13 https://artofproblemsolving.com/community/c1621835_2019_
Tags: combinatorics