Swapping places!

Algebra Level 3

When Tom is about going to school in the afternoon, it's over 1 p.m. When he reaches school, he realizes the two hands of the clock have already swapped its position. If the two hands of the clock never overlapped from the time Tom started to go to school to the time Tom reached school, how long did it take Tom to go to school? (unit: hour)

If the answer is in the form a b \frac{a}{b} where a a and b b are coprime positive integers, type a + b a+b

Bonus What time was it when Tom started to go to school and reached school?


The answer is 25.

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2 solutions

The hour hand moves 2 π radian in 12 hours The minute hand moves 2 π radian in 1 hour Let ‘t’ be the time taken in hours for Tom to get to school,in time t both the hands combined have moved 2 π radians 2 π 12 × t + 2 π 1 × t = 2 π t 12 + t = 1 t = 12 13 hours \text {The hour hand moves }2\pi \text{ radian in} \text{ 12 hours}\\ \text {The minute hand moves }2\pi \text{ radian in} \text{ 1 hour}\\ \text{Let `t' be the time taken in hours for Tom to get to school,in time t both the hands combined have moved } 2\pi \text{ radians}\\ \begin{aligned}\implies \dfrac{2\pi}{12}\times t+\dfrac{2\pi}{1}\times t&=2{\pi} \\ \implies \dfrac{t}{12}+t&=1\\ \implies t&=\color{#EC7300}\boxed{\color{#333333}\dfrac{12}{13}} \color{#333333} \text{ hours}\end{aligned}

Let when Tom started from home, the minute hand of the clock made an angle x degrees with the hour hand, while the later made an angle y with the 1-hr mark. Then (360-x)/12=x or x=360/13. (30+x+y)/12=y or y=(30+x)/11=750/143. Therefore the minute hand made an angle of (9000/143) degrees with the 12-hr mark, and hence Tom started for school at 1hr (1500/143) min. When he reached school, the hands of the clock swapped. Therefore the time was then 2 hr (840/143) min, and hence he took (132/143) or (12/13) hour to reach the school.

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