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Question
A motorcyclist drives from A to B with a uniform speed of 30 km h–1 and returns back with a speed of 20 km h–1. Find its average speed.
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Solution
Let the distance between A and B be S km.
Time taken in going from A to B = `"s"/30 "h"`
Time taken in going from B to A = `"s"/20 "h"`
Average speed of motorcyclist = `"total distance covered"/"total time taken"`
= `("S km" + "S km")/("S"/30"h" + "S"/20"h")`
= `2/(1/30 + 1/20) "km h^-1"`
= `2/(1/12) "km h"^-1`
= 24 km h−1
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