$\frac{mass}{length}$=$\frac{20 g}{m}$
mass$=18\,g$
length of string $=\frac{18}{20} m =\frac{9}{10} m$
from diagram $\frac{\lambda}{2}=\ell$
$\Rightarrow \lambda=2 \ell=\frac{9}{5}\,m$
again speed $v=f \lambda=50 \times \frac{9}{5}=90\,m / s$
(Take speed of sound in air as $340\, {ms}^{-1}$ )
Given, $F 1= F 2=500\, Hz$. speed of air $=330\, m / s$