The combined volume is 320 cu. in.
The volume of the bottom of the box is given by the formula
V = lwh
In this situation, we have
V=8(8)(4) = 256 cu. in.
The volume of the top is given by the formula
V = 1/3lwh
For this situation, we have
V=1/3(8)(8)(3) = 64 cu. in.
Together, this makes
256+64 = 320 cu. in.