You want to mail a gift that is in the shape of a rectangular prism with the dimensions 25 cm by 32 cm by 40 cm .what is the minimum amount of brown paper you need to cover the gift?

Check the picture below.

notice, the gift box is just 6 rectangles stacked up to each other at the edges,

left and right, two rectangles of 32x40

top and bottom, two rectangles of 25x32

front and back, two rectangles of 25x40

so, just get the area of all 6 rectangles, add them up, and that's the "surface area" of the rectangular prism, and that's how much paper will be needed to cover it.