Answer
Mass fraction= 0.308
Weight percent= 30.8%
Work Step by Step
Mass fraction of $CS_{2}$=
$\frac{mass\,of \,CS_{2}}{mass\,of\,CS_{2}+mass\,of\,CHCl_{3}}=\frac{20.0\,g}{20.0\,g+45.0\,g}$
$=0.308\,$
Weight percent= mass fraction$\times100\%$
$=30.8\%$
