title: 'Problem 189: Tri-colouring a triangular grid'
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## Description
<sectionid='description'>
Consider the following configuration of 64 triangles:
We wish to colour the interior of each triangle with one of three colours: red, green or blue, so that no two neighbouring triangles have the same colour. Such a colouring shall be called valid. Here, two triangles are said to be neighbouring if they share an edge.