A motorboat travelling at some speed can cover 28 km upstream and 40 km downstream in 11 hours. At the same speed it can travel 30 km downstream and 16 km upstream in 7 hours, then the speed of the stream is:

Correct option is C

Speed of the boat in still water = b km/h
Speed of the stream = s km/h
In first case
​$$\frac {28} { (b - s)} + \frac {40}{ (b + s)} = 11$$​​
In second case
​$$\frac {30 }{ (b + s)} + \frac{16 }{(b - s)} = 7$$​​
On  Solving these two equations:
b - s = 4
b + s = 10
On subtracting both the equations , s = 3km/hr