Question

A person rowing at the rate of 5 km/h in still water, takes thrice as much as time in going 40 km upstream as in going 40 km downstream. Find the speed of the stream.

Answer 1

Let’s assume x to be the speed of the stream. 

So, we know that 

Speed of boat in downstream = (5 + x) and, 

Speed of boat in upstream = (5 – x) 

It is given that, 

The distance in one way is 40 km. 

And, 

Time taken during upstream = 3 × time taken during the downstream 

Expressing it by equations, we have 

40/ (5 – x) = 3 x 40/ (5 + x) [∵ time = distance/ speed] 

By cross multiplication, we get 

(5+x) = 3(5-x) 

⇒ 5 + x = 3(5 – x)

⇒ x + 3x = 15 – 5 

⇒ x = 10/4 = 2.5 

Therefore, the speed of the stream is 2.5 km/hr.