Question

speed of a boat in still water is 15km/hr. It goes 30 km upstream and returns back at the same point in 4 hr 30 min. Find the speed of the stream

Answer 1

Speed of boat = 15 km/hr
Let speed of stream = y km/hr
Downstream speed= 15 + y
Upstream speed = 15 - y
Given,
30/(15+y) + 30/(15-y) = 4 hrs 30 mins = 4 1/2 hrs = 9/2 hrs
30[ (1/15+y + 1/15-y)] = 9/2
15+y+15-y/(15+y)*(15-y) = 9/2*30
30/225-y^2 = 3/2*10
30*2*10 = 3*(225-y^2)
10*2*10 = 225 - y^2
200 = 225 - y^2
y^2 = 25
y = 5
Hence, speed of stream = 5 km/hr
Hope it helps.

Answer 2

Hey there!

Let speed of stream = x km/h

∴ Downstream speed = (15+x) km/h

∴ Upstream speed = (15-x) km/h

∴ 30/(15+x) + 30/(15-x) = 4 1/2

→ 900/225-x² = 9/2

→ 9x² = 225

→ x² = 25

→ x = 5

∴ Speed of stream = 5 km/h

Hope it Helps You!