A motor boat heads upstream a distance of 24 km in a river whose current is running at 3
km per hour. The trip up and back takes 6 hours. Assuming that the motor boat maintained
a constant speed, what was its speed in still water?
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Let the speed of the boat in still water be x km/hr.
Speed of boat in upstream = x - 3 km/hr
and in downstream = x + 3 km/ hr.
So 24/(x+3) + 24/(x-3) = 6
=> [24(x-3) + 24(x+3)]/(x+3)(x-3) = 6
=> 48x = 6(x² - 9)
=> 8z = x² - 9 => x² -8x -9 = 0
= (x-9)(x+1) = 0 => x = 9 or x = -1
x cannot be negative . So x = 9
Speed of boat = 9km/hr
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