IKMC 2027 - Grades 7-8 - Self-study SET 3 - Thematic Batch 3 Question 13

A 6 × 6 square grid is shown in Figure 1. What is the minimum number of cells that must be coloured so that the 3 × 1 rectangle in Figure 2 cannot lie entirely within uncoloured cells?

Editorial note: As a grid-cell placement problem, the rectangle follows grid lines and may be horizontal or vertical.