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

Quantitative Aptitude ( CCS) Boats and Streams

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

4.5 km/hr

8 km/hr

5.5 km/hr

2 km/hr

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Explanation:

Suppose he moves 5km downstream in x hours
Then, downstream speed a= 5/x km/hr
Speed upstream speed b = 4/x km/hr
40 / (5 /x) + 40 / (4/x) = 18
8x + 10x = 18
x = 1
a = 5 km/hr, b = 4 km/hr
speed of boat = ½ (5 + 4 ) = 9/2 km/hr=4.5