Question

The speed of a river current is 4 mph. IF a boat travels 20 miles downstream in the same time that it takes to travel 10 miles upstream, find the speed of the boat in still water.
downstream speed = u + 4
upstream speed = u - 4

20 = (u+4) t
10 = (u-4)t

so
20/(u+4) = 10/(u-4)

10 u + 40 = 20 u - 80

10 u = 120
u = 12 miles/hr

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