Clock Conundrum

Geometry Level 2

To the nearest second, what is the earliest time after midnight (excluding 00:00:00) that the minute hand and hour hand are precisely on top of one another?

01:05:00 01:05:27 01:05:12 01:00:00

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1 solution

Isaac Reid
Dec 7, 2015

Firstly, note that the first time the hands cross after 00:00:00 will come between 1 'o' clock and 2 'o' clock. Hence, we can take 1 'o' clock as a starting point in any calculations of distance travelled by the hands.

The hour hand moves at a rate of 5 x / h r 5x/hr , where 1 x 1x is an increment of 1 minute of the clock. Meanwhile, the minute hand moves at a rate of 60 x / h r 60x/hr . However, since we already know that the point of overlap comes after 1 'o' clock, we can simplify the problem by immediately giving the hour hand it's 5x headstart: see the picture attached if this isn't clear.

v = d / t v=d/t , so using these previously calculated "rates", we can formulate expressions for the distance travelled by each hand. The hour hand's distance (i.e. number of increments) from the 1 'o' clock position is given by 5 t + 5 5t +5 whereas the minute hand's position is given by 60 t 60t . These can be solved simultaneously: 60 t = 5 t + 5 60t=5t+5 55 t = 5 55t=5 t = 5 55 = 1 11 t=\frac{5}{55}=\frac{1}{11} So the hands overlap at 1 11 \frac{1}{11} of a minute - 27 seconds - after 01:05:00. So the solution is 01 : 05 : 27 \boxed{01:05:27} .

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