The rows, columns and two long diagonals in this chart all add up to 34.

Now let's suppose that you are forbidden to use the numbers 2 and 15, but allowed in their place to repeat any two numbers already in use, how would you construct your square so that rows, columns and diagonals should still add up to 34?

Your ability to achieve this will depend on the two numbers that you select to replace 2 and 15

If for the 2 and 15 you substitute 7 and 10, repeated, the square can be formed as shown.

Any sixteen numbers can be arranged to form the magic square if they can be written in this way, so that all the horizontal differences are alike and all the vertical differences are also alike.