A boat goes 45 km upstream in 6 hours and 60 km downstream in 4 hours. If the speed of the current is doubled, how much time will the boat take to travel 90 km downstream?

Let the speed of the boat in still water be B km/h and the speed of the stream be S km/h. From the given data: Upstream speed = B - S = 45 km / 6 hours = 7.5 km/h. Downstream speed = B + S = 60 km / 4 hours = 15 km/h. Solving these two equations: B - S = 7.5 and B + S = 15. Adding the two, 2B = 22.5 → B = 11.25 km/h. Substituting B = 11.25 into B + S = 15: S = 3.75 km/h. Now, the speed of the current is doubled, so the new speed of the stream is 2 * 3.75 = 7.5 km/h. The new downstream speed = 11.25 + 7.5 = 18.75 km/h. Time to travel 90 km downstream = 90 / 18.75 ≈ 4.8 hours.