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): A tugboat goes 120 miles upstream in 20 hrs.
the return trip downstream takes 5 hrs.
find the speed of the tugboat without a current and the speed of the current
Let s = boat speed in still water
Let c = speed of the current
(s-c) = effective speed upstream
(s+c) = effective speed downstream
Write distance equation for each way. (dist = time * speed
20(s-c) = 120
5(s+c) = 120
Simplify both equations, divide the 1st equation by 20, divide the 2nd equation by 5, results:
s - c = 6
s + c = 24
-----------Adding eliminates c find s
2s = 30
s = 30/2
s = 15 mph boat speed in still water
You can find the current, check the solutions in the original equations