①:=x: ②-0.5x= ③=
x-x=15 40%:x=0.2:4.5 25(x+13)=650
[1-(+)]÷ 2÷× (8.9-7.7)÷0.2×0.15
+ x=2 8∶x =∶ 50%x-9×3=25
3.8×99+3.8 2.8+4.36+5.64 [×(-)]÷