Choosen As Best Answer

假設船速為 x (km/hr) => 順流時的速度 = x+6 (km/hr),逆流時的速度 = x-6 (km/hr)
花費的時間要換成小時,3小時20分 = 10/3 (hr)

48/(x+6) + 48/(x-6) = 10/3
(48x -48*6 +48x +48*60 / (x^2 -36) = 10/3
96x/(x^2 -36) = 10/3
10x^2 -360 = 288x
10x^2 -288x -360 = 0
5x^2 -144x -180 = 0
(x -30)(5x +6) = 0
x = 30 or -6/5(不合)

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