a motor -cycle covers 40 Km with a speed of 20 km /hr. Find the speed of the motor -cycle for the next 40 km journey so that the average speed of the whole journey will be 30 km / hr.

A 70.0km /hr
B 52.5km /hr
C 60.0km/hr
D 60. 5 km / hr

Correct Answer: 60.0km/hr

Let the desired speed be x km/hr ATQ, 40/20 + 40/x = 80/30 Or, 1/20 + 1/x = 2/30 Or, 1/x = 1/15 - 1/20 Or, 1/x = (4 - 3)/60 Or, x = 60 So, the desired speed is 60 km/hr
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