wt of solute $=30\,gm$

Volume of solution $=100\,mL$

wt. of solution $=1.2 \times 100=120\,gm$

wt. of solvent $=120-30=90\,gm$

$m =\frac{30 \times 1000}{180 \times 90}=1.85$

$\frac{24- P _{ s }}{ P _{ s }}=\frac{1.85 \times 18}{1000}$

$24- P _{ s }=0.0333 P _{ s }$

$P _{ s }(1.033)=24$

$P _{ s }=23.22$