When the Clock Hands Meet
At : PM the minute and hour hands of a standard clock coincide. After how many hours do they next coincide? Give an exact fraction if the answer is not a whole number of hours.
Work it out, then compare with the answer.
SolutionShow the worked solution
Measure angles in laps and time in hours. The minute hand turns at lap per hour and the hour hand at lap per hour. The minute hand therefore gains on the hour hand at
The hands start together. They next coincide when the minute hand has gained exactly one full lap. That takes
Equivalently, in degrees, , which gives .
This is hour and minutes, so the meeting is at about 1:05:27.
Generalisation. The minute hand gains a lap every hours. The hands therefore coincide at times hours after noon, for . That is meetings in every hours, not .