The speed of a current in a body of water is 1 mile per hour. Jill swims 4 miles against the current and 2 miles with the current in a total time of 1 hour and 40 minutes. How fast can Jill swim in still water?

let the speed of current be Vw =1, and that of jill be V, then we have

4/(V-Vw) +2/(V+Vw) = 5/3

2/(V-Vw) +1/(V+Vw) = 5/6

2(V+Vw)+V-Vw = 5/6 (V^2-Vw^2)

5V^2-18V -11 =0

V = [9+sqrt(81+55)]/5

= 4.13 mph

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