Answer:
5/(3+c) +5/(3-c) = 3
Explanation:
The current adds to Milly's rowing speed going downriver, so her speed downriver is (3+c). The relationship between speed, time, and distance is ...
... time = distance/speed
so her time going downriver is 5/(3+c).
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The current subtracts from Milly's rowing speed gowing upriver, so her speed upriver is (3-c). Her time going 5 miles upriver is 5/(3-c).
We are told her time for both trips is 3 hours, so we have ...
... 5/(3+c) +5/(3-c) = 3
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Comment on the equation
This equation has no solution in real numbers. The minimum time for the distance corresponds to c=0, and that time is 3 1/3 hours. There is no real value of c that will let Milly make the trip in 3 hours.