The average age of the boys in a class is 12 years. The average age of the girls in the class is 11 years. There are 50% more girls than boys in the class. Which one of the following is the average age of the class (in years)?
Average age of boys ($A_B$) = 12 years.
Average age of girls ($A_G$) = 11 years.
$N_G$ is 50% more than $N_B$, so $N_G = N_B + 0.50 N_B = 1.5 N_B$.
The total sum of ages of boys is $N_B \times A_B = 12N_B$.
The total sum of ages of girls is $N_G \times A_G = 11N_G = 11(1.5N_B) = 16.5N_B$.
The total sum of ages in the class is $12N_B + 16.5N_B = 28.5N_B$.
The total number of students is $N_B + N_G = N_B + 1.5N_B = 2.5N_B$.
The average age of the class is $\frac{\text{Total sum of ages}}{\text{Total number of students}} = \frac{28.5N_B}{2.5N_B} = \frac{28.5}{2.5} = \frac{285}{25}$.
$\frac{285}{25} = \frac{57 \times 5}{5 \times 5} = \frac{57}{5} = 11.4$.
The average age of the class is 11.4 years.