（100-x）/x=25%可化为（100-x）/x=1/4进而（100-x）*4=x400-4x=x400=5xx=80
（100-x）/x=25% 100-x=25%xx+0.25x=1001.25x=100x=100÷1.25x=80
(100-x）/x=25%(100-x）/x=1/4x=400-4x5x=400x=80
