A ridge tent has a uniform cross-section shaped like an isosceles triangle with a base of 6 m and a height of 4 m, and the tent runs 12 m from front to back. How much air, in cubic metres, does it enclose?

The tent is a prism, so its volume = (area of the triangular cross-section) $x$ length = $(1/2)\times 6\times 4\times 12=12\times 12=144$ cubic m. The trap 288 drops the 1/2 from the triangle's area; 96 applies a pyramid's 1/3 to 288, but a prism keeps the same cross-section all along and needs no 1/3; 72 halves the correct value once more.