Question
A motor-cycle covers 160 km with a speed of 70 km/hr.
find the speed of the motor-cycle for the next 170 km journey so that the average speed of the whole journey will be 90 km/hr.Solution
Average speed of whole journey = 90 km/hr Using the formula, average speed = 2xy/(x + y)  x = speed of first 160 km = 70 km/hr 90 = 2 x (70 x y)/(70 + y) = 126 km/hr
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