x+3=15 解:x+3-3=15-3 x=12 x÷=12÷ x=36 6∶5=x∶ 解:5x=6× 5x=2 5x÷5=2÷5 x= x-37.5%x=400 解:(1-37.5%)x=400 x=400÷0.625 x=640 x-20%x=160 解:80%x=160 x=160÷0.8 x=200 x-×= 解:x=+ x=÷ x=1 :x=5:21 解:5x=×21 x=14÷5 x=