A man can row 60 km upstream and 100km downstream in same time. If the speed of boat in still water is 20 km / hr, find the speed of stream?

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A man can row 60 km upstream and 100km downstream in same time. If the speed of boat in still water is 20 km / hr, find the speed of stream?
1. 7 km / hr
2. 8 km / hr
3. 5 km / hr
4. 9 km / hr
5. 10 km / hr

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Correct Answer - Option 3 : 5 km / hr

Given

Distance travelled upstream = 60 km

Distance travelled upstream = 100 km

The speed of boat in still water is 20 km / hr.

Formula used∶

Let u and v be the speed of boat in still water and speed of stream respectively.

Speed upstream = (u - v)

Speed downstream = (u + v)

Calculations

Let the speed of stream be x.

Speed upstream = (20 - x) km / hr

Speed downstream = (20 + x) km / hr

Time taken to travel upstream = $\frac{{60}}{{20 - {\rm{x}}}}hr$         ---(1)

Time taken to travel downstream = $\frac{{100}}{{20 - {\rm{x}}}}hr$         ---(2)

According to the question time taken in travelling upstream and downstream is same;

On equating (1) and (2) we get x = 5km / hr

Hence the speed of stream is 5 km / hr