If you cut it into three sections according to the section and cut it twice, the bottom surface (section) will increase by three and the surface area will increase by 36 square decimeters.
The cross-sectional area is 36÷4=9 square decimeters =0.09 square meters.
Then the volume of the original cuboid wood is cross-sectional area × height =0.09×5=0.45 cubic meters.