Kangaroos

From the Netherlands Junior Maths Olympiad:

There is a kangaroo in the centre of each square of a 5-by-5 grid of squares. Lightning strikes, and each kangaroo simultaneously jumps one square up, down, left or right. What is the greatest possible number of empty squares that could remain?

Solution

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Elastic Numbers

It’s here! My new book of puzzles Elastic Numbers is now available on Amazon.

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Elastic Numbers Cover

I hope you enjoy solving the problems as much as I enjoyed creating them.