One third of the box will be 3 inches long, so is equivalent to a 3-inch cube. It has a volume of
... (3 in)³ = 27 in³
The appropriate choice is ...
... D. 27 cu. in.
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Volume is found by multiplying length by width by height. When all are the same (3 in), the product is (3 in)·(3 in)·(3 in) = (3 in)³.