A boat takes half time in moving a certain distance downstream than upstream. What is the ratio between the rate in still water and rate of current?

A. 1 : 2
B. 3 : 1
C. 2 : 1
D. 1 : 3
Explanation:
Let the speed of the boat in still water be 4 km/hr and speed of the current be u km/hr.
Rate downstream = (u + v) km/hr.
Rate upstream = (u- v)km/hr
Let the distance covered in each case be x km.
Then 2x/(u + v) = x / (u – v)
=> 2 (u – v) = ( u + v)
=> u =3v
=> u/v =3/1