A boat takes 4.0 h to travel 36 km down a river, then 5.0 h to return. how fast is the river flowing

Let us refer to boat speed as X and water speed as Y On the way down the stream, the water speed is added to the speed of the boat: speed of the boat (X) + water speed (Y) is 9km/h When the boat is moving upstream, water speed is taken away from the speed of the boat: Speed of the boat (X) - water speed (Y) is 7.2 km/h Therefore: X + Y = 9 : X - Y = 7.2 2X = 16.2 X = 8.1 X + Y = 9 8.1 + Y = 9 Y = 0.9 The river is flowing at 0.9 km/h