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Speed of boat in still water is 7.5 km/hr and stream speed is 1.5 km/hr and is travelling upstream. It has a log of wood attached which decreases the speed of boat by 20%. Log of wood is detached somewhere in between and it comes back to the shore in 12 hours, Find the total time(in hour) for boat to reach other shore if total width in km is 55.

Speed of boat in still water is 7.5 km/hr and stream speed is 1.5 km/hr and is travelling upstream. It has a log of wood attached which decreases the speed of boat by 20%. Log of wood is detached somewhere in between and it comes back to the shore in 12 hours, Find the total time(in hour) for boat to reach other shore if total width in km is 55. Correct Answer 10(1/6)

Given:-

Speed of boat = 7.5 km/hr

Speed of stream = 1.5 km/hr

Width of river = 55 km

Time for log to come back = 12 hours.

Formula:-

Speed = Distance/Time

Downstream speed = u + v

Upstream speed = u - v

U and v are boat and stream speeds.

Calculation:-

Speed of boat with log of wood attached = 80% of 7.5 = 6 km/hr

Upstream speed = 6 - 1.5 = 4.5 km/hr

Downstream speed without log = 7.5 - 1.5 = 6 km/hr.

Distance travelled by log of wood(to return) = 12 × 1.5 = 18 km

Total time for boat = 18/4.5 + (55 - 18)/6 = 4 + 6(1/6) = 10(1/6) hours.

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