The volume of a rectangular prism is its length times width times height, or algebraically,
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. You may be used to computing volume with numbers, but remember, a variable is a stand-in for a number. So you can solve this in the same way. Substitute
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into the formula for volume. You get
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, and you multiply these factors together. As you would with ordinary fractions, multiply the numerators and denominators across. You get
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. It seems that the book wants you to simplify by bringing the 6 up to the denominator. Recall the rule
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, if n is non-negative. The opposite applies so that
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. For your final answer, you write
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. This corresponds to
answer choice B.