The speed of a boat in still water is 10 km/hr. If it can travel 26 km downstream and 14 km upstream in the same time, the speed of the stream is:
Solution
Correct Answer: Option C
Let the rate of the stream be x km/h. The boat's rate when the river speeds up the boat by adding its speed of x km/h to the boat's speed giving 10+x km/h. The boat's rate when the river slows the boat down and subtracts its speed of x km/h from the boat's speed giving 10-x km/h. Time = Distance/Rate Downstream time =(Downstream Distance)/rate = 26/(10+x) Upstream time = (Upstream Distance)/rate = 14/(10-x) Those times are equal. 26/(10+x) = 14/(10-x) Might as well divide both sides by 2 13/(10+x) = 7/(10-x) 13(10-x) = 7(10+x) [Cross-multiply] 130-13x = 70+7x -20x = -60 x = 3 km/h
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