Let the speed of boat be B and the speed of stream be S. ⇒ Speed of the boat in downstream = 60/3 = 20 km/hour ⇒ Speed of the boat in upstream = 90/5 = 18 km/hour ⇒ Speed of the boat = (20 + 18)/2 = 19 km/hour ⇒ Speed of the stream = (20 – 18)/2 = 1` km/hour ⇒ Speed of the boat : Speed of the stream = 19 : 1 ∴ Required ratio is 19 : 1.