The radius of a circle is increased by 10%.

Use the successive formula $x + y + \frac{xy}{100}$ where $x$ and $y$ are both $10\%$. Effectively, $10 + 10 + \frac{10 \times 10}{100} = 21\%$.

Many students wrongly assume the area increase is equal to the radius increase (10%), forgetting that area is a two-dimensional property ($A = \pi r^2$).

This same 'successive change' method applies to any problem involving a product of two variables, such as calculating changes in volume of a cube (three-dimensional) or total bill amounts with two successive discounts.