The moment of inertia of a hollow cubical box of mass M and side a about an axis passing through the centres of two opposite faces is equal to

Taking mass of plate m       Then MI of two plates through which axis is passing  = m a26×2 = ma23M.I. of 4 plates having symmetrical position from the axis.= 4×[ma212+m(a2)2]  = 4×[ma23]Total MI = 4 ma23+ma23 = 5ma23using  m =M6 ⇒MI = 5 Ma218