The long, straightforward, brute-force way to slog through the problem:
The prism is 12cm x 6cm x 9cm.
Volume of the prism = (12 x 6 x 9) = 648 cm³
Volume of each little cube = (1.5 x 1.5 x 1.5) = 3/375 cm³
Number of little cubes that fit into the prism = (648 cm³) / (3.375 cm³) =
192 of them
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The elegant but almost equally long way to master the problem:
The prism is 12cm x 6cm x 9cm.
If each dimension of your measuring cubie is 1.5cm,
then the prism measures
(8 cubie lengths) x (4 cubie widths) x (6 cubie heights) .
Its volume is (8 x 4 x 6) = 192 measuring cubies.