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.

From the given bar graph, the blue bars represent the total number of students (Boys + Girls) and the orange bars represent the difference between the number of boys and girls (Boys - Girls) in schools A, B, C, and D.

For any school, if the number of boys is denoted by B and the number of girls by G, we have the following relations:

$B+G=\text{Total Students (Blue Bar)}$

$B-G=\text{Difference between Boys and Girls (Orange Bar)}$

Subtracting the second equation from the first equation, we get:

$2G=\left(\text{Boys + Girls}\right)-\left(\text{Boys - Girls}\right)$

$G=\frac{\left(\text{Boys + Girls}\right)-\left(\text{Boys - Girls}\right)}{2}$

Now, let us calculate the number of girls in schools A, C, and D step-by-step using the data points visible in the image:

1. School A:

From the chart for School A, total students (Blue bar) = 72, and difference (Orange bar) = 48.

$\text{Girls in A}=\frac{72-48}{2}=\frac{24}{2}=12$

2. School C:

From the chart for School C, total students (Blue bar) = 48, and difference (Orange bar) = 12.

$\text{Girls in C}=\frac{48-12}{2}=\frac{36}{2}=18$

3. School D:

From the chart for School D, total students (Blue bar) = 52, and difference (Orange bar) = 4.

$\text{Girls in D}=\frac{52-4}{2}=\frac{48}{2}=24$

Calculating the Average Number of Girls in A, C & D:

$\text{Total Girls in A, C \& D}=12+18+24=54$

$\text{Average Number of Girls}=\frac{54}{3}=18$

Thus, the average number of girls in schools A, C, and D is 18.