3550.24 – Put Your Hands Together


At what precise time after 12 noon, do the two hands of a clock overlap for the first time?


Solution

The hands will cross sometime after 1 where the dashed line in diagram below indicates. To be more precise, let xx be the distance past 1 when this happens. We measure distances around the circular face of the clock in degrees. Then 30°30° is the distance between hours and x/30°x/30° is the fraction of the hour (between 1:00 and 2:00) that the hour hand traverses to meet the minute hand. The minute hand, in the same time, meanwhile traverses the fraction (x+30)/360(x + 30)/360 of the whole clock-circle. These fractions must be equal for the hands to meet, therefore

x30=x+30360.\frac{x}{30} = \frac{x+30}{360}.

Solving this equation gives x=30/11x = 30/11. In other words, the hands meet one eleventh of an hour past 1. In minutes this is 60/11=5.454545...60/11 = 5.454545 . . . or 5 minutes and about 27 seconds past 1.

How many times in 12 hours do the hands meet (besides the moment at the beginning when both are vertical)? Does this suggest another approach to the problem?

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