Let the speed of the boat in still water be x km/hour and the speed of the current be y km/hour. Distance covered by the boat downstream in 2 hours = 20 km ∴(x+y)×2=20 ⇒x+y=10....(1) Distance covered by the boat upstream in 2 hours = 4 km ∴(x−y)×2=4 ⇒x−y=2.....(2) Subtracting (2) from (1), we have 2y=8 ⇒y=4 Thus, the speed of the current is 4 km/hour.