A boat takes 3 hours to cover 120 km downstream and takes 6 hours to cover the same distance upstream. Then what distance it will cover running for 4 hours in still water?
Calculation Let the speed of boat in still water and the speed of the stream respectively = x km/hour and y km/hour Speed of the boat while going downstream = (x + y) km/hour ⇒ (x + y) =
120
3
= 40 km/hour Speed of the boat while going upstream = (x - y) km/hour ⇒ (x – y) =
120
6
= 20 km/hour Adding both speed of the boat ⇒ (x + y) + (x – y) = 40 + 20 ⇒ 2x = 60 ⇒ x = 30 km/hour So the speed of the boat in still water = 30 km/h Distance covered by the boat in 4 hours in still water = 30 × 4 = 120 km ∴ Distance covered by the boat in 4 hours in still water is 120 km.