A gas is expanded from an initial state to a final state along a path on a \(P\)-\(V\) diagram. The path consists of (i) an isothermal expansion of work \(50\;J\), (ii) an adiabatic expansion and (iii) an isothermal expansion of work \(20\;J\). If the internal energy of gas is changed by \(-30\;J\), then the work done by gas during adiabatic expansion is