Question: At six o’clock, a spider starts to walk at a constant speed from the hour hand anticlockwise round the rim of the clock face. When it reaches the minute hand, the spider turns around and walks round the rim in the opposite direction at the same constant speed, reaching the minute hand again after a further 20 minutes. What time does the clock read when the spider reaches the minute hand for the second time?
Solution: After the spider begins the second stage of its journey, 10 minutes pass before the end. In that time the minute hand moves through 120˚. The spider thus moves through 480˚. This implies that the spider is moving four times as fast as the minute hand. During the first stage of the journey, let the minute hand move through x˚. This spider thus moves through (180 - x)˚ in the same time. Because the spider is four times as quick as the minute hand, we have 180 - x = 4x and so x = 36. Hence the total angle that the minute hand sweeps out is 36˚ + 120˚ = 156˚, which corresponds to 26 minutes. Therefore at the end of the spider's journey the clock reads 6:26.