Rahman is a boatman. He can row a boat at the speed of 5 km/hr upstream and 15 km/hr downstream. Find the speed of the stream.

Question: Rahman is a boatman. He can row a boat at the speed of 5 km/hr upstream and 15 km/hr downstream. Find the speed of the stream.

Solution:
Let’s denote:
B as Speed of the boat in still water (km/h)
S as Speed of the stream (km/h)

Speed of the boat upstream is the speed of the boat in still water minus the speed of the stream:
B - S = 5 km/hr -------- (1)

Speed of the boat downstream is the speed of the boat in still water plus the speed of the stream:
B + S = 15 km/hr -------- (2)

(1) + (2)
B - S = 5
B + S = 15
2B = 20
∴ B = 10 km/hr

Putting the value of B in (2)
∴ S = (15 - 10) km/hr = 5 km/hr

∴ The speed of the stream is 5 km/hr