Dig it out of the corner? Constant surface area: 20x20x6=2400? Because this square has six faces, it is multiplied by six.

Dig in the middle Surface area increase: 20x20x6+10x10x5 = 2900? After digging out the middle, there are five small squares with the side length of 10, which need to be added with 10x 10x5.

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