Question

The speed of a boat in still water is 6 km/h and the speed of the stream is 1.5 km/h. A man rows to a place at a distance of 22.5 km and comes back to the starting point. Find the total time taken by him
1. 5 hrs 
2. 4 hrs 
3. 7 hrs 
4. 8 hrs 
5. None of these

Answer 1

Correct Answer - Option 4 : 8 hrs 

Given 

Speed of boat in still water = 6 km/h

Speed of stream = 1.5 km/h

Distance taken = 22.5 km

Formula Used

If speed of a boat in still water is u km/h and the speed of the stream is v km/h. Then,

Speed upstream = (u - v)km/h

Speed downstream = (u + v) km/h

Calculation

Boat’s upstream speed = 6 – 1.5 = 4.5 km/h

Boats downstream speed = 6 + 1.5 = 7.5 km/h

⇒Total time = (22.5/4.5) + (22.5/7.5) = 5 + 3 = 8 hrs.

∴ Total time is 8 hrs.