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cube of side 5is divided into 125 black and white unit cubes

Source: 1981 Swedish Mathematical Competition p4

March 28, 2021
combinatorial geometrycombinatorics

Problem Statement

A cube side 55 is divided into 125125 unit cubes. NN of the small cubes are black and the rest white. Find the smallest NN such that there must be a row of 55 black cubes parallel to one of the edges of the large cube.