If the length, width, and height of the little cubies are 1/3 inch,
then the volume of each cubie is
(1/3 inch) x (1/3 inch) x (1/3 inch) = 1/27 inch³ .
If the box holds exactly 567 of them, then the volume
of the box is
(567) x (1/27 inch³)
= (567 x 1 / 27) inch³
= (567 / 27) inch³ = 21 inch³ .