Two security patrol cars, Car A and Car B, are circling a perimeter road around an airport. They start from the same checkpoint at 5:00 AM, but in opposite directions. Car A completes 3 laps per hour and Car B completes 2 laps per hour. How many times do the two cars pass each other between 5:00 AM and 6:30 AM?
(a) 7
(b) 5
(c) 6
(d) 8

Question

Two security patrol cars, Car A and Car B, are circling a perimeter road around an airport. They start from the same checkpoint at 5:00 AM, but in opposite directions. Car A completes 3 laps per hour and Car B completes 2 laps per hour. How many times do the two cars pass each other between 5:00 AM and 6:30 AM?
(a) 7
(b) 5
(c) 6
(d) 8

ONLiNE UPSC · Accepted Answer

Relative speed when moving in opposite directions = 3 + 2 = 5 laps/hour
Time = 1.5 hours &rarr; Total crossings = 5 &times; 1.5 = 7.5
So they must pass each other 7 times in that duration. (the 8th time would be after 6:30pm)
Hence, option a is correct.

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M Question 1	⏱ 0
Two security patrol cars, Car A and Car B, are circling a perimeter road around an airport. They start from the same checkpoint at 5:00 AM, but in opposite directions. Car A completes 3 laps per hour and Car B completes 2 laps per hour. How many times do the two cars pass each other between 5:00 AM and 6:30 AM? (a) 7 (b) 5 (c) 6 (d) 8