The average sale of a car dealership was 15 cars per week. After a promotional scheme the average sale increased to 21 cars per week. The percentage increase in the sale of cars was

### Concept & Formula Percentage increase measures the growth of a quantity relative to its initial value. $$ \text{Percentage Increase} = \left( \frac{\text{Final Value} - \text{Initial Value}}{\text{Initial Value}} \right) \times 100 $$ ### Step-by-Step Solution * **Given:** * Initial average sales = $15$ cars/week * Final average sales = $21$ cars/week * **Calculation:** * Find the absolute increase: $21 - 15 = 6$ cars. * Apply the percentage increase formula: * $\text{Percentage Increase} = (\frac{6}{15}) \times 100$ * Simplify the fraction: $\frac{6}{15} = \frac{2}{5}$ * $\text{Percentage} = \frac{2}{5} \times 100 = 40\%$ ### Exam Strategy & Shortcut Familiarize yourself with standard fraction-to-percentage conversions. Knowing that $\frac{1}{5} = 20\%$ allows you to instantly recognize that $\frac{2}{5} = 40\%$ without writing down the multiplication step. ### Common Pitfall Calculating the percentage increase over the *final* value instead of the *initial* value. Always put the difference over the original starting number (i.e., $\frac{6}{15}$, not $\frac{6}{21}$). ### Final Answer **Therefore, the correct answer is 40%.**