A person row a boat 12 km downstream in 1 hour. If the river current flows at 3 km/h, find the time taken by the boat to row the same distance upstream.
Explanation:
Downstream speed = 12 / 1 = 12 km/h. Baseline boat speed in still water = Downstream speed - Current speed = 12 - 3 = 9 km/h. Upstream speed = Still water speed - Current speed = 9 - 3 = 6 km/h. Time taken upstream = Distance / Upstream speed = 12 / 6 = 2 hours.
A field clerk walking at 5 km/h handles a regular sector. If he steps up to run at 10 km/h, he takes 24 minutes less across the course stretch. Find the absolute distance profile.
A motorboat covers 12 km upstream and returns back to the starting jetty in 3 hours. If the speed of the boat in still water is 9 km/h, find the speed of the river current.
Explanation:
Let the speed of the current be c km/h. 12 / (9 - c) + 12 / (9 + c) = 3. Dividing by 3: 4 / (9 - c) + 4 / (9 + c) = 1 => 4(9 + c + 9 - c) = 81 - c^2 => 72 = 81 - c^2 => c^2 = 9 => c = 3 km/h.
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