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in Distance, Time and Speed by (49.2k points)
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A boatman rows to a place at a distance of 45 km and back in 20 hours. He finds that he can row 12 km with the stream in the same time as 4 km against the stream. Find the speed of the stream. 

(a) 3 km/hr 

(b) 2.5 km/hr 

(c) 4 km/hr 

(d) 3.5 km/hr

1 Answer

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Best answer

(a) 3 km/hr

Ratio of upstream speed to downstream speed = 1 : 3 

∴ Ratio of times taken to cover a certain distance upstream and downstream = 3 : 1 

Given, the man took 20 hours to cover the 45 km distance and back  ⇒  He took 15 hours to cover 45 km upstream and 5 hours to the cover 45 km downstream.

∴ Speed downstream = \(\frac{45}{5}=9\) km/hr

Speed upstream = \(\frac{45}{15} = 3\) km/hr

∴ Speed of stream = \(\frac12(9-3)\) km/hr = 3 km/hr.

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