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