The volume of wood required to make a closed box of thickness 2.5 cm with external dimensions $$90 cm \times 75 cm \times 50 cm$$ is:

Given,

Length,L=90cm, Breadth,B=75cm, Height,H=50cm

Thickness=2.5cm

Volume of Outer box=$$L\times\ B\times\ H=90\times\ 75\times\ 50$$

Volume of Outer box=337500 $$cm^3$$

To find the volume of inner box  subtract thickness

new Dimensions are

$$L'=90-5=85cm$$ Thickness=2.5cm

B'=75-5=70cm

B'=50-5=45cm

Volume of  inner  box=$$L'\times\ B'\times\ H'=85\times\ 70\times\ 45$$
Volume of inner  box=267750 $$cm^3$$
Volume of  wood=Volume of outer  box-Volume of inner box
Volume of wood=337500-267750=69750 $$cm^3$$