As you move downwards, numbers increase by 3, then 2, then 1, before repeating this pattern.

The numbers in each row of the diagram add up to 18.

The sum of the 4 digits at the corners of the diagram is 17, as is the sum of the 4 digits in the middle of each side of the diagram.

Starting with the top left square, and moving clockwise around the other 3, the sum of the digits in each square follows the sequence 20, 19, 18, 17.

As you move clockwise around the diagram, add the previous 2 numbers together and subtract 3 to give the next number in the chain.