There are 12 girls and 9 boys in a group.

1. Find the probability of selecting a boy and simplify it.

Total number of people $=12+9=21$.

Number of boys $=9$.

$P(\text{boy})=\frac{9}{21}=\frac{3}{7}$

1. Let the number of more boys needed be $x$ and form the probability equation.

Let $x$ be the number of more boys needed.

Then the number of boys becomes $9+x$, and the total number of people becomes $12+9+x=21+x$.

$\frac{9+x}{21+x}=\frac{4}{5}$

2. Solve the equation for $x$.

$5(9+x)=4(21+x)$
$45+5x=84+4x$
$x=39$