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

Answer: Option B

Number of cars sold per week originally = 15 × 7 = 105
Number of cars sold per week under scheme = 21 × 7 = 147
Increase in sale = 147 - 105 = 42
∴ Percentage increase
$$\begin{array}{rl}& =\left(\frac{42}{105}\times 100\right)\%\\ & =\left(\frac{4200}{105}\right)\%\\ & =40\%\end{array}$$