A marquee is a right pyramid on a square base of side 24 m, and each of its four triangular faces has a slant height of 20 m. How much canvas, in square metres, is needed for the sloping sides?

Each sloping face is a triangle with base 24 m and slant height 20 m, giving $(1/2)\times 24\times 20=240$ sq m, so four faces need $4\times 240=960$ sq m. The trap 1536 adds the $24\times 24=576$ sq m base, but a marquee pitched on the ground needs no canvas underneath; $1920=4\times 24\times$ 20 drops the 1/2 from each triangle; 768 uses the VERTICAL height instead of the slant height — that height is √$(20^2-12^2)=16$ m, and $4\times (1/2)\times 24\times 16=768$, which measures the inside of the tent rather than its skin.