Question
A boat can travel 80 km downstream in 8 hours, and it
can travel the same distance upstream in 10 hours. What is the speed of the boat in still water?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
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