Rakesh rows to a place 40 km distant and back in total of 18 hours He finds that he can row 5 km wit...........

Quant (SSC) Boats and Streams

Rakesh rows to a place 40 km distant and back in total of 18 hours. He finds that he can row 5 km with the stream in same time as 4 km against the stream. What is the speed of boat in still water?

3.0 km/hr

4.5 km/hr

5 km/hr

6 km/hr

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Explanation:

Suppose he moves 5 km downstream in t hours
Then, downstream speed a= 5/t km/hr
Speed upstream speed b = 4/t km/hr
40 / (5 /t) + 40 / (4/t) = 18
8t + 10t = 18
t = 1
a = 5 km/hr, b = 4 km/hr
speed of boat = ½ (5 + 4 ) =4.5 km/hr