Inert Content for a Variable Volume Reaction

An irreversible gas phase reaction \(A \rightarrow 5B\) is conducted in an isothermal batch reactor at constant pressure in the presence of an inert. The feed contains no \(B\). If the volume of the gas at complete conversion must not exceed three times the initial volume, the minimum mole percent of the inert in the feed must be _______ (choose from: 0, 20, 33, 50)