A four-sided field has a diagonal 40 m long. From the two opposite corners, the perpendiculars dropped onto this diagonal measure 9 m and 13 m. What is the area of the field, in square metres?

The diagonal splits the field into two triangles that share it as their common base, so the area = $(1/2)\times 40\times 9+(1/2)\times 40\times 13=(1/2)\times 40\times (9+13)=440$ sq m. The trap $880=40\times 22$ forgets the 1/2; 220 halves the correct value again, as though only one triangle counted; 80 uses the difference 13 - 9 in place of the sum, which would suit a trapezium's height, not two offsets on the same base.