Always 20!There are nine disks in this puzzle. Each disk contains four digits. There is a number in the empty space between any four disks. The number is equal to the sum of the digits adjacent to the space from the respective disks  just as shown in the illustration below.
The goal is to rotate the disks in order to make each of the four numbers equal 20 as soon as possible. This is always possible. A disk can be rotated in any direction either quarter of turn or half of turn. Just place mouse cursor over the disk and the arrows will show the angle and direction of the rotation. Click the respective arrow to initiate the rotation. If you complete a challenge: Good Luck!
