A
Don't forget that you can also pull out an x besides the 10. The expression should look like this.
A(x) = 10x(x - 5)
The GCF is 10x
B
The area of the Bedroom = 10x ( x - 5)
If the length = 5x then the width is
A/L = 10x (x - 5)/(5x)= 2*(x - 5)
C
Now they are changing the Length to the GCF. We determined in A that the GCF = 10x
This time A/L = 10x(x - 5)/10x = (x - 5)
So the width = x - 5