Answer
Mass fraction= 0.0284
Weight percent= 2.84%
Work Step by Step
Mass of benzene= $density\times volume$
$=0.877\,g/mL\times4.00\,mL=3.508\,g$
Mass fraction of benzene=
$\frac{mass\, of\, benzene}{mass\,of\, benzene+mass\,of \,diethyl\,ether}=\frac{3.508\,g}{3.508\,g+120.\,g}=0.0284$
Weight percent= mass fraction$\times 100\%$
$=2.84\%$
