Discussion
A and B are running on a circular track of length 600 m. Speed of A is 30 m/s and speed of B is 20 m/s. They start from the same point at the same time in the same direction. After how much time will they meet at the starting point?
Answer: C
They meet at the starting point after 60 seconds.
A completes a lap in 600/30 = 20 s and B in 600/20 = 30 s. Both are at the start together whenever the time is a multiple of both lap times:
\[ \text{LCM}(20, 30) = 60\ \text{s} \]
In 60 s A runs 3 laps and B 2, so both are back at the start. 40, 50 and 70 seconds are not multiples of both lap times; at 40 s, for example, A is at the start but B is two-thirds of the way round.
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