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.
Let the distance (AB) = x
Total distance = x + x = 2x
Speed = distance/time
t1 = x/30
t2 (return trip) = x/20
Total time taken = t1 +t2
Total time taken = x/30 + x/20
= (2x +3x)/60
= 5x/60
= x/12
Average speed = total distance/total time taken
= 2x / 
= (2x) x 
= 24 km/hr
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