The speed of the boat in still water is 20 km/hrs and speed of the stream is 5 km/hr. Find the time taken to travel 50 km downstream or 30 km upstream by the boat.

4 hours

Speed of boat (u) = 20 km/hr

Speed of current (v) = 5 km/hr

Speed of boat in downstream =20+5=25

Speed of boat in upstream =  20-5=15

Now,

Time to go 50km downstream =\(\frac{50}{25}\) =2

Time to go 30km upstream = \(\frac{30}{15}\) = 2

Total time taken = 2+2 = 4hr