The white dot moves two places anti-clockwise at each stage and the black dot moves one place clockwise at each stage around the corners of the figure. After how many stages will they be together in the same corner? Answer with a natural number.
What number should replace the question mark? Three figures each have a top, left, middle, right and bottom number: (7, 9, 3, 6, 2), (5, 4, 2, 6, 1) and (13, 19, ?, 11, 5). Answer with a natural number.
Switch A turns lights 1 and 2 on-off or off-on. Switch B turns lights 2 and 4 on-off or off-on. Switch C turns lights 1 and 3 on-off or off-on. White = ON, black = OFF. Switches A, C and B are thrown in turn, with the result that Figure 1 turns into Figure 2. Which of the switches A, B or C must be faulty?