A square field has a boundary 800 m long. What is its area in hectares? (1 hectare = 10000 square metres.)

The boundary is the perimeter, so the side = 800/4 = 200 $m$ and the area = $200^2=40000$ sq $m=40000/10000=4$ hectares. The trap 16 halves the perimeter instead of quartering it, giving a side of 400 $m$; 40 divides the 40000 sq $m$ by 1000; 0.4 divides by 100000.