Starting with the numbers in the top row, and following straight lines through the centre of the diagram, subtract the middle number from the top number to give the corresponding value on the bottom row.

In each circle, starting at the top left segment, numbers increase, as you move clockwise, by 2 for the upper left circle, 3 for the upper right, 4 for the lower right and 5 for the lower left.

In each group of 3 numbers, the lower number equals the average of the top two numbers.

Reading each row as a 3 digit number, the rows follow the sequence of square numbers, from 17 to 19.

Reading each row as 3 separate 2-digit numbers, the central number equals the average of the left and right hand numbers.