ContourPlot[1=(y*0.3048*10/7*9.8/1.83^2-100/x)*0.45,2=(y*0.3048*10/7*9.8/1.83^2-100/x)*0.45,3=(y*0.3048*10/7*9.8/1.83^2-100/x)*0.45,{x, 1, 100}, {y, 4, 15}]

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