\(P V=\) 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 \((ii)\)
\(\frac{2 P_0}{3}+20=P_0 \Rightarrow P_0=60 cm \text { of } Hg .\)