length = 6 x 1/3 = 2 inches
width = 4 x 1/3 = 4/3 inches
height = 8 x 1/3 = 8/3 inches
volume = lwh = 2 x 4/3 x 8/3 = 64/9 cubic inches
Another approach is the following:
There are 6 x 4 x 8 = 192 cubes inside the prism,.
Each cube has a volume of 1/27 cubic inches.
Therefore total volume = 192 x 1/27 = 192/27 = 64/9