The bar shows the total number of students in four schools and it also shows the difference between boys and girls in these schools.

Find the average number of girls in A, C & D.

The correct answer is 18.

To find the average number of girls in schools A, C, and D, we first need to determine the number of girls in each of these schools using the data provided in the bar graph.

For any school, let $B$ represent the number of boys and $G$ represent the number of girls.
The blue bar represents the total number of students, which is:
$B+G$
The orange bar represents the difference between boys and girls, which is:
$B-G$

We can solve for the number of girls ($G$) by subtracting the difference from the total and dividing by 2:
$G=\frac{(B+G)-(B-G)}{2}$

Let's calculate the number of girls for schools A, C, and D using the values visible in the graph:

1. School A:
Total students ($B+G$) = 72
Difference ($B-G$) = 48
$G_A=\frac{72-48}{2}=\frac{24}{2}=12$

2. School C:
Total students ($B+G$) = 48
Difference ($B-G$) = 12
$G_C=\frac{48-12}{2}=\frac{36}{2}=18$

3. School D:
Total students ($B+G$) = 52
Difference ($B-G$) = 4
$G_D=\frac{52-4}{2}=\frac{48}{2}=24$

Average Number of Girls in A, C & D:
We calculate the average by summing the number of girls in these three schools and dividing by 3:
$\text{Average}=\frac{12+18+24}{3}$
$\text{Average}=\frac{54}{3}=18$