A boat covered a distance of 15 km upstream in 5 hours and a distance of 42 km downstream in 6 hours. The speed of the stream in km/h is:

2

Let the speed of the boat = x and the speed of the stream = y

Boat covers 42 km in 6 hours downstream

In one hour = 7 km distance covered downstream

Boat covers 15 km in 5 hours Upstream 

Upstream speed = 3 km per hour

According to the concept =

Downstream =

x + y = 7----(A)

upstream =

x - y = 3----(B)

From A and B we get,

x = 5 and y = 2

So the speed of the stream = 2