Problem 32.36 Easy Free

When the Clock Hands Meet

At 1212:0000 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 11 lap per hour and the hour hand at 112\frac{1}{12} lap per hour. The minute hand therefore gains on the hour hand at

1−112=1112 lap per hour.1-\frac{1}{12}=\frac{11}{12}\ \text{lap per hour}.

The hands start together. They next coincide when the minute hand has gained exactly one full lap. That takes

t=111/12=1211 hours.t=\frac{1}{11/12}=\frac{12}{11}\ \text{hours}.

Equivalently, in degrees, 360t=30t+360360t=30t+360, which gives t=1211t=\frac{12}{11}.

This is 11 hour and 55115\frac{5}{11} minutes, so the meeting is at about 1:05:27.

1211 hours≈1.09 hours\boxed{\dfrac{12}{11}\ \text{hours}\approx 1.09\ \text{hours}}

Generalisation. The minute hand gains a lap every 1211\frac{12}{11} hours. The hands therefore coincide at times 12k11\frac{12k}{11} hours after noon, for k=0,1,2,…k=0,1,2,\dots. That is 1111 meetings in every 1212 hours, not 1212.