A boat can travel 80 km downstream in 8 hours, and it

Solution

Let the speed of the boat in still water be x km/h, and the speed of the stream be y km/h. Downstream speed = x + y Upstream speed = x - y We know: Time downstream = Distance / Speed = 80 / (x + y) = 8, so x + y = 10 Time upstream = Distance / Speed = 80 / (x - y) = 10, so x - y = 8 Now, solve these two equations: x + y = 10 x - y = 8 Add the two equations: 2x = 18 ⇒ x = 9 km/h So, the speed of the boat in still water is 9 km/h. Correct Option: c) 9 km/h