Any Triangle End

A common mistake with a tent is to use a slanted side as the height. The height of the triangle end is the dashed line. It goes from the top straight down to the base, at a right angle.

The area of the end is half the base times this height: $\frac{1}{2}\cdot 8\cdot 3 = 12$. The tent is 10 long, so it holds $12\cdot 10 = 120$. The slanted sides are not used.