**From New Scientist #2807, 9th April 2011** [link]

Joe printed and cut out nine 3 by 3 cardboard magic squares. The nine squares used all the numbers from 1 to 81.

For a square to be magic, the sums of the numbers in each row, each column and each diagonal must all be the same. This sum is called the magic constant.

Joe gave the nine magic squares to Penny and suggested she arrange them to make a 9 by 9 magic square and tell him the magic constant of the 3 by 3 square she placed in the middle.

What was the magic constant?

[enigma1641]

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The following Python code runs in 200ms.

Solution:The magic constant on the central 3×3 square is 123.