There are 4 more girls in Mrs. Chang's class than Mr. Blackwell's class. Five girls moves from Mr. Chang's class to Mr. Blackwell's class. Now there are twice as many girls in Mr. Blackwell's class as there are in Mrs. Chang's. How many girls were in Mr. Blackwell's class to begin with?

Mr. B. Class: X $Girls.

Mr. C. Class:: (x+4) Girls.

Mr. B. Class: (x+5).
Mr. C. Class: (x+4) - 5 = (x-1).

(x+5) = 2(x-1)
x+5 = 2x - 2
x-2x = -2 -5 = -7
-x = -7
X = 7 Girls to begin with.

Let's break down the information given step by step:

1. Let's assume the number of girls in Mr. Blackwell's class to be x.
2. Since there are 4 more girls in Mrs. Chang's class, the number of girls in Mrs. Chang's class is (x - 4).
3. After 5 girls move from Mr. Chang's class to Mr. Blackwell's class, the number of girls in Mr. Chang's class becomes (x - 5), and the number of girls in Mr. Blackwell's class becomes (x + 5).
4. Now, it is given that there are twice as many girls in Mr. Blackwell's class as in Mrs. Chang's class, so we can write the equation: (x + 5) = 2 * (x - 4).

Now, let's solve the equation to find the value of x, which represents the number of girls in Mr. Blackwell's class to begin with:

x + 5 = 2x - 8
Adding 8 to both sides:
x + 13 = 2x
Subtracting x from both sides:
13 = x

Therefore, there were 13 girls in Mr. Blackwell's class to begin with.

To find the number of girls in Mr. Blackwell's class to begin with, we can work through the problem step by step.

Let's assume the number of girls in Mr. Blackwell's class initially is "x".

1. There are 4 more girls in Mrs. Chang's class than Mr. Blackwell's class, so the number of girls in Mrs. Chang's class is "x + 4".

2. Five girls move from Mr. Chang's class to Mr. Blackwell's class, so the number of girls in Mr. Blackwell's class is now "x + 5".

3. After the movement of girls, there are twice as many girls in Mr. Blackwell's class as there are in Mrs. Chang's class. This gives us the equation (x + 5) = 2(x + 4).

To solve this equation and find the value of x:

Expand the equation: x + 5 = 2x + 8
Subtract x from both sides: 5 = x + 8
Subtract 8 from both sides: -3 = x

So, the initial number of girls in Mr. Blackwell's class (x) was -3. However, since we cannot have a negative number of students, it means that there must be an error in the problem or the information provided. Please double-check the question or provide additional information if available.