1 ()-9 () = 1 () 1 ()-99 () = 1 () 1 ()-999 () = 1 ()

1 (meter) -9 (decimeter) = 1 (decimeter) 1 (decimeter) -9 (centimeter) = 1 (centimeter) 1 (centimeter) -9 (millimeter) = 65.

1 (yuan) -9 (angle) = 1 (angle) 1 (angle) -9 (minute) = 1 (minute)

1 (m) -99 (cm) = 1 (cm) 1 (decimeter) -99 (mm) = 1 (mm)

1 (square kilometer) -99 (hectare) = 1 (hectare) 1 (square centimeter) -99 (square decimeter) = 1 (square decimeter) 1 (square decimeter) -99 (.

1 (yuan) -99 (min) = 1 (min)

1 (km) -999 (m) = 1 (m) 1 (m) -999 (mm) = 1 (mm)

1 (ton) -999 (kg) = 1 (kg) 1 (kg) -999 (g) = 1 (g)

* I don't know if I learned the following in the third grade of primary school *

1 (cubic meter) -999 (cubic decimeter) = 1 (cubic decimeter) -999 (cubic centimeter) = 1 (cubic centimeter) -999 (cubic centimeter)