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TOT 2014 Spring A J2, two player game in 5x5 board

Source:

September 11, 2020
gamecombinatoricssquare table

Problem Statement

Peter marks several cells on a 5×55\times 5 board. Basil wins if he can cover all marked cells with three-cell corners. The corners must be inside the board and not overlap. What is the least number of cells Peter should mark to prevent Basil from winning? (Cells of the corners must coincide with the cells of the board).