To find the volume of the box Mary makes you must first multiply the length and breadth of her starting rectangle (1500cm) then multiply that number by 3, because the box will be 3 cms high. (4500ccms). However, by folding up the sides, Mary has made the box 3 cm shorter in both length and breadth. So the internal area will be 15cm - 3 cm on both long sides (6 cms) = 9 cms.
Then 10cms - 3 cms on both short sides = 4 cms. We find the cubic centimetres of this internal area, and it's 9 x 4 = 36 x 3 cms height, which is 109 ccms. We then must subtract the internal section from the former larger one to find the actual volume.
