A rectangular field is 0.5 km long and 300 m wide. Find its area in hectares. Take 1 hectare = $10000{\text{m}}^2$.

Put both sides in metres first: 0.5 km = 500 m, so the area = $500\times 300=150000{\text{m}}^2$. Dividing by 10000 gives 15 hectares. (150 divides by 1000, 1500 divides by 100 and 1.5 divides by 100000 — the divisor is 10000, because a hectare is a square of side 100 m and $100^2=10000.$)