Let the number of boys = \( 5x \), girls = \( 8x \)
After changes:
Boys = \( 5x - 5 \), Girls = \( 8x + 4 \)
\[
\frac{5x - 5}{8x + 4} = \frac{1}{2}
\Rightarrow 2(5x - 5) = 8x + 4
\Rightarrow 10x - 10 = 8x + 4
\Rightarrow 2x = 14 \Rightarrow x = 7
\]
So, Boys = 35, Girls = 56
Difference = \( 56 - 35 = 21 \)