Question

A man, rowing a boat, covers a distance of 21 km in 7 hours downstream. He covers (1/3)rd distance in same amount of time when rowing upstream. Calculate the speed of the stream.
1. 1 km/h
2. 4 km/h
3. 5 km/h
4. 7 km/h

Answer 1

Correct Answer - Option 1 : 1 km/h

Given

Down stream distance = 21 km

Time taken in downstream = 7 hours

Distance covered in upstream = (1/3) × 21

= 7 km

Time taken in upstream = 7 km

Formula used

Speed of stream = (Downstream speed - Upstream speed)/2

Speed = Distance/Time

Calculation

Downstream speed = 21/7 = 3 km/h

Upstream speed = [21 × (1/3)]/7 = 7/7

⇒ 1 km/h

Speed of stream = (Downstream speed - Upstream speed)/2

⇒ (3 - 1)/2 = 2/2

⇒ 1 km/h

∴ Speed of stream is 1 km/h