Raju travels 150 km in 7.5 hours moving downstream. The speed of the stream is 5 km/h. Find the speed with which Raju should row the boat to reach back in the same time.?

Option 4 : 25 km/h

Given:

Distance = 150 km

Time = 7.5 hours

Speed of the stream = 5 km/h

Concept used:

Speed = Distance/time

Downstream = u + v

Upstream = u - v

u = speed of the boat in still water

v = speed of the stream

Calculation:

Downstream speed of the boat = 150/7.5

⇒ 20 km/h

Here,

u + v = 20

⇒ u + 5 = 20

⇒ u = 15

∴ speed of the boat in still water = 15 km/h

Now,

Raju needs to cover the same distance in 7.5 hours upstream

Upstream speed should be 20 km/h

∴ u - v = 20

⇒ u - 5 = 20

⇒ u = 25

The speed of the boat in still water should be 25 km/h

∴ Raju should row the boat at 25 km/h to reach back at the same time.