A cube has been divided into $27$ equal-sized sub-cubes. We take a line that passes through the interiors of as many sub-cubes as possible. How many does it pass through?
Problem
Source: 2019 Dürer Math Competition Finals Day2 E9 E+4 https://artofproblemsolving.com/community/c1621835_2019_
Tags: combinatorics, combinatorial geometry