FILE No.83Unsolved
A safe door has a 3 x 3 grid of nine squares. Each square holds one of the numbers 1 to 9, used once. The door opens when every row, every column and both diagonals add up to 15.
You know the top row is 2, 7, 6, and the bottom-left square is 4.
What number is in the centre?
If you get stuck, open the hints one at a time, in order.
Look for a diagonal line.
Do the top-right and bottom-left squares lie on a line through the centre?
What is the sum of that line?
What number is in the centre?
Choose the answer closest to the truth.
5
The answer is: 5.
The top-right square (6), the centre and the bottom-left square (4) lie on one diagonal. 6 + centre + 4 = 15, so the centre is 5. The completed square is 2 7 6 / 9 5 1 / 4 3 8. The centre of a 3 x 3 magic square is always 5.
Looking only at the top row and left column, you can miss the diagonal clue.
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