Can you read the time?

Geometry Level 5

Let O O be the centre of a standard clock face and let T T be the point at the 12 o'clock position. Let A A be the end point of the hour-hand and B B the end point of the minute-hand. At some time between 10:00 o'clock and 11:00 o'clock, accurate to the nearest second, A O T = B O T \angle AOT= \angle BOT and this can be represented as X X hours, Y Y minutes, Z Z seconds.

Find the smallest possible value of X + Y + Z X+Y+Z .


The answer is 33.

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1 solution

Let the angular speeds of hour-hand and minute-hand by ω h \omega_h and ω m \omega_m respectively, then we have ω h = 3 0 60 = 1 2 /min \omega_h = \dfrac {30^\circ}{60} = \frac 12 ^\circ \text{/min} and ω m = 36 0 60 = 6 /min \omega_m = \dfrac {360^\circ}{60} = 6^\circ \text{/min} .

At 10:00 o'clock, A O T = 6 0 \angle AOT = 60^\circ and B O T = 0 \angle BOT = 0^\circ . Let the required B O T A O T \angle BOT-\angle AOT be θ \theta and the time after 10:00 to reach the required angles be t t minutes, then for minute-hand: θ = ω m t = 0.5 t \theta = \omega_m t = 0.5 t and hour-hand: θ = 60 ω h t = 60 6 t \theta = 60 - \omega_h t = 60 - 6t , then:

t 2 = 60 6 t t = 120 12 t 13 t = 120 t = 120 13 min 9 min 14 s \begin{aligned} \frac t2 & = 60-6t \\ t & = 120 - 12t \\ 13 t & = 120 \\ \implies t & = \frac {120}{13} \text{ min} \approx 9 \text{ min } 14 \text{ s} \end{aligned}

The time is therefore 10 : 9 : 14 10:9:14 , X + Y + Z = 10 + 9 + 14 = 33 \implies X+Y+Z = 10+9+14 = \boxed{33}

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