Question

The speed of a boat in still water is 30 km / h. It goes 60 km in the opposite direction of the current of the stream and returns to the starting point in the direction of the current of the stream. In this entire journey, it has taken 4 hours 10 minutes. How much time (in hours) will it take to cover a distance of 136.8 km towards the flow of the water stream?
1. 2.4
2. 3.8
3. 4.2
4. 3.2

Answer 1

Correct Answer - Option 2 : 3.8

Given:

Speed of boat in still water = 30kmph

Formula:

Downstream speed = Speed of boat + speed of stream

Upstream speed = Speed of boat - Speed of stream

Speed = Distance/time

Calculation:

Total time = 4 hours and 10 min = 25/6 hours

Let speed of stream be a kmph.

Downstream speed = 30 + a

Upstream speed = 30 - a

But,

⇒ 60/(30 + a) + 60/(30 - a) = 25/6

⇒ a = 6 kmph

Speed of stream is 6 kmph.

Downstream speed = 30 + 6 = 36kmph

∴ Required time = 136.8/36 = 3.8 hours