Kept Apart

The squares of this grid contain one of the letters P, Q, R and S. Can you complete this square so that touching squares do not contain the same letter? How many possibilities are there?
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Problem



In each of the squares in the grid, one of the letters P, Q, R and S must be entered in such a way that touching squares (whether connected by an edge or just a corner) do not contain the same letter. Some of the letters have alread been entered as shown.

What are the possibilities for the letter in the shaded square?

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Kept Apart


If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.