Question

A boat travel 640 km upstream and downstream each. Speed of boat in still water is 24 km/h. Time taken by boat to travel upstream is two times the time taken by boat to travel downstream. Find the speed of stream.
1. 8 km/h
2. 5 km/h
3. 4 km/h
4. 6 km/h

Answer 1

Correct Answer - Option 1 : 8 km/h

Given

Distance travelled = 640 km

Speed of boat = 24 km/h

Formula used

Time = Distance/Speed

Downstream speed = Speed of boat + Speed of stream

Upstream speed = Speed of boat - Speed of stream

Calculation

Letthe speed of the stream is s km/h

Downstream speed = 24 + x

Upstream speed = 24 - x

According to question

2[640/(24 + x)] = 640/(24 - x)

2/(24 + x) = 1/(24 - x)

48 - 2x = 24 + x

48 - 24 = x + 2x

24 = 3x

8 = x

∴ The speed of stream is 8 km/h