A boat can travel 21 km in 3 hours going upstream. It can travel 55 km in 5 hours going downstream. What is the speed of the stream?

2 km/h

Let the speed of the boat and stream is x and y respectively.

 A boat can travel 21 km in 3 hours going upstream.

= x - y = \(\frac{21}{3}\)

= x - y = 7 km/hr ...........(A)  

It can travel 55 km in 5 hours going downstream. 

 = x + y = \(\frac{55}{5}\)

= x + y = 11km/hr.............(B)

Subtracting Equation (B) to equation(A)   

= 2y = 4 

Speed of the stream (y) = 2