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UKMT Junior Kangaroo, 2019

Combinatorics Difficulty 2.0 Junior Find the answer United Kingdom

Sid is colouring the cells in the grid using the four colours red, blue, yellow and green in such a way that any two cells that share a vertex are coloured differently. He has already coloured some of the cells as shown.

What colour will he use for the cell marked $X$ ?

(A) Red (B) Blue (C) Yellow (D) Green (E) You can't be certain

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