Explanation

(a)
Let molality of solution = x
Moles of solute in 1000 g benzene
= $\frac{1000}{78}$ =12.82
Mole fraction of solute = $\frac{x}{x + 12.82}$
0.2=$\frac{x}{x + 12.82}$
or 0.2(x+12.82)=x
or 0.2x+2.564 = x
2.564 = x - 0.2x
$x = \frac{2.564}{0.8}$ = 3.2