A boat has a speed of 9 mph in still water. If the boat takes the same time to go 28 miles upstream as it takes to go 44 miles downstream, what is the speed of the current?

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The river flows downstream at c mph, so the boat’s upstream speed is reduced by the speed of the current to 9-c, while its downstream speed is increased to 9+c. Time t taken to go upstream, t=28/(9-c) and downstream t=44/(9+c). Therefore 28/(9-c)=44/(9+c), 28(9+c)=44(9-c) by cross-multiplying, 252+28c=396-44c, 72c=144, so c=144/72=2mph.

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