Exhibit: It All Adds Up
Place the balls in the holes in the circular base so that, the three numbers along each diameter line add up to the same total. There are eight depressions in the tray around the outside of the circle and one in the middle, so there are four diameters outlined in red across the circle. The numbers 1 to 9, with one removed to go into the middle, have to be divided up into four pairs that add up to the same total. There are three ways of doing this; removing either of the lowest numbers, the highest number or the middle value (1, 9 or 5). This leaves it possible to have a run of four small numbers which can be paired with four larger ones.