A circular jar lid has a circumference of 66 cm. Find the area of the lid. Use π = $\frac{22}{7}$ (answer in ${\text{cm}}^2$).

First get the radius: $66=2\times \frac{22}{7}\times r$ → $r=\frac{66\times 7}{44}=10.5$ cm. Then $A=\frac{22}{7}\times 10.5^2=\frac{22}{7}\times 110.25=346.5{\text{cm}}^2$. Using the diameter 21 cm as the radius gives the trap 1386; forgetting to square $r$ gives 33; 693 is twice the correct area.