The time taken by a boat to cover a distance of x km upstream is double the time taken by the boat to cover the same distance downstream. The sum of the speeds of the boat and the stream is 90 km/h. The speed of the stream is _____________ km/h.

Correct option is A

Let the speed of boat be a km/hr and speed of stream be b km/hr.

Speed of boat and Speed of stream = a+ b = 90 km/hr........(1)

​$$\frac{x}{a-b}= 2(\frac{x}{a+b})\\a-b = 45.............(2)$$​​

Solve equation 1 and 2, we get 

b = 22.5 km/hr