The boat and the soccer ball
A rower is rowing upstream, on the Saint-John river, at a constant speed with respect to the water. Just as he passes the Princess Margaret bridge his soccer ball falls in the water. He continues upstream for 20 minutes before noticing that he has lost his soccer ball. He immediately reverses , rows downstream and finds his soccer ball 2 km below the bridge. How fast is the river flowing?
Give your answer in km/h. The river is flowing at a constant speed.