Question

The ratio of the speed of the boat in still water and the speed of the current is 5 : 1. It can go 9 km upstream in 2 hours 30 minutes and return to the starting point. What is the speed of the stream in km/h?
1. 2.5
2. 1
3. 2
4. 1.5

Answer 1

Correct Answer - Option 4 : 1.5

Given:

Ratio of speed of boat in still water and speed of stream = 5 : 1

The boat cover 9 km in upstream in 2 hour 30 minutes

Formula used:

Downstream = speed of boat + speed of stream

Upstream = speed of boat - speed of stream

Calculation:

Let the speed of boat and speed of stream be 5x km/h and x km/h respectively

So,

Speed in upstream

9/(5x - x) = 9/4x hours

Speed in downstream

9/(5x + x) = 3/2x hours

Now

(9/4x) + (3/2x) = 2.5

⇒ (9 + 6)/4x = 2.5

⇒ x = 15/(2.5 × 4) = 1.5 km/h

∴ The speed of the stream is 1.5 km/h