b
(b)
$P V=$ constant
$P_1 V_1=P_2 V_2 \quad\left[P_1=P_0\right.$ atmospheric pressure $]$
$P_0 \times 40=P_1 \times 60 \quad \ldots (i)$
$P_1+20=P_0 \quad... (ii)$
From $(i)$
$P_1=\frac{2 P_0}{3}$
From $(ii)$
$\frac{2 P_0}{3}+20=P_0 \Rightarrow P_0=60 cm \text { of } Hg .$