The diagonal of a square field measures 40 m. What is its area in square metres?

For a square of diagonal $d$ the area is $d^2/2=1600/2=800$ sq m, because the side is 40/√2 and not 40/2. The trap 1600 treats the diagonal as the side; 400 takes the side to be half the diagonal, 20 m; 3200 multiplies by 2 where it should divide.