Question

A race-boat covers a distance of 60 km downstream in one and a half hour. It
covers this distance upstream in 2 hours. The speed of the race-boat in still
water is 35 km/hr. Find the speed of the stream.​

Answer 1

Step-by-step explanation:

Let the speed of the stream be x km/h

Speed of the boat = 35km/h

Distance covered by the boat = 60km

Speed of boat downstream = (35 + x)km/h

Given, time taken by the boat to go downstream = 1 ½ hr = 3/2 hrs

As, Time = $\rm \dfrac{Distance}{Speed}$

→ $\rm \dfrac{3}{2} = \dfrac{60}{35 + x}$

By cross multiplication:-

→ 3(35 + x) = 2(60)

→ 105 + 3x = 120

→ 3x = 120 - 105

→ 3x = 15

→ x = 5km/h

Let us check for upstream:-

Distance = 60km

Speed of boat upstream = (35 - x)km/h

Time taken to go upstream = 2hrs

Time = $\rm \dfrac{Distance}{Speed}$

→ $\rm 2= \dfrac{60}{35 - x}$

→ 2(35 - x) = 60

→ 70 - 2x = 60

→ 2x = 70 - 60

→ 2x = 10

→ x = 5km/h

∴ Speed of the stream = 5km/h

Answer 2

Hope you will understand