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Choose four of the numbers from this list: $1, 2, 3, 4, 5, 6, 7, 8, 9$ to put in the squares below so that the difference between joined squares is odd.
Only one number is allowed in each square. You must use four different numbers.
What can you say about the sum of each pair of joined squares?
What must you do to make the difference even?
What do you notice about the sum of the pairs now?

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This problem is based on an idea taken from "Apex Maths Pupils' Book 2" by Ann Montague-Smith and Paul Harrison, published in 2003 by Cambridge University Press. To find out more about this book, and order a copy go to the CUP website.