Question 49

A motor-boat cantravel at 10 km/hour in stillwater. Ittravelled91 km downstream in a river and then returned to the same place, taking altogether 20 hours. Find the rate of flow ofriver

Solution

motor-boat can travel at 10 km/hour in still water

distance covered in upstream and downstream = 91 km

$$ speed = \frac{distance}{time} $$

given, rate of still water = 10 km/hr

assume rate of stream = x

then downstream speed = x + 10

upstream speed = x - 10

total time = 20 hrs

according to the question

$$ 20 = \frac{91}{10 - x} + \frac{91}{10 + x} $$

solving, x = 3 km/hr

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