The appropriate choice is ...
(25−2x)(20−2x)x
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When x is cut from each side of the 25 dimension, the remaining dimension of the bottom of the box is 25-2x. Similarly, when x is cut from each side of the 20 dimension, the remaining dimension of the bottom of the box is 20-2x. The length of the side that is folded up is x, so that is how deep the box is.
The volume is the product of the dimensions of the bottom and the side:
(25−2x)(20−2x)x
