Question

Speed of the stream is 20% of the boat, if it covers 120 km upstream in 90 minutes then speed of the boat in downstream is:

1. 120 km/h
2. 100 km/h
3. 80 km/h
4. None of these

Answer 1

Correct Answer - Option 1 : 120 km/h

Given:

Stream = 20% of speed of boat

Concept used:

Upstream = u – v

Downstream = u + v

Here u = speed of the boat

v = stream

Calculation:

Let speed of the boat be 5x

So, speed of the stream is 5x × 1/5 = x

u – v = 5x – x = 4x

According to the question,

120/4x = 90

⇒ 30/x = 3/2

⇒ x = 20

u + v = 5x + x

⇒ 100 + 20

⇒ 120

⇒ Downstream speed = 120 km/h

∴ Speed of the boat in downstream is 120 km/h