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Let

,

, and

be the length width and height of Charlie's box.
So the volume of Charlie's box is
Also, the surface area of Charlie's box is

Factor out the GCF 2.
--------------------
Now let

,

, and

be the length width and height of Lucy's box.
Since Lucy's box has " dimensions twice as large as Charlie's", this means that

,

, and
So the volume of Lucy's box is

Now plug in

,

, and

Multiply
So the volume of Lucy's box is 8 times larger than Charlie's box.
Also, the surface area of Lucy's box is

Factor out the GCF 2.

Now plug in

,

, and

Multiply

Factor out the GCF 4 and multiply it by the outer term 2
So the surface area of Lucy's box is 8 times the surface area of Charlie's box. So she needs 8 times more paint than Charlie.
So simply multiply the amount of paint that Charlie uses (one half gallons) by 8 to get
So she needs 4 gallons of paint to cover her box.