Each dial has one hand, and the dot at the top marks the 12 o'clock position. How is the third dial of each row related to the first two? Find the missing dial.

Measure how far each hand has turned clockwise from 12. In each row the third hand has turned as far as the first two together: 3 o'clock plus 3 o'clock gives 6 o'clock; 1 o'clock plus 4 o'clock gives 5 o'clock. In the bottom row 2 o'clock plus 7 o'clock gives 9 o'clock, so the hand points left: (a). (b) takes the difference, (c) points to 8 o'clock and (d) copies the second dial.