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-

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
$$\eqalign{ & = \left( {\frac{{42}}{{105}} \times 100} \right)\% \cr & = \left( {\frac{{4200}}{{105}}} \right)\% \cr & = 40\% \cr} $$