The elementary liquid-phase irreversible reactions
$A\overset{k_{1}=0.4\: min^{-1}}{\rightarrow}B\overset{k_{2}=0.1\:min^{-1}}{\rightarrow}C$
take place in an isothermal ideal $CSTR$(Continuous-Stirred Tank Reactor). Pure $A$ is fed to the reactor at a concentration of $2$ mol/Liter. For the residence time that maximizes the exit concentration of $B$, the percentage yield of $B$, defined as $\left [ \frac{net\:formation\:rate\:of\:B}{consumption\:rate\:of\:A} \times100\right ]$ , is ________________ (rounded off to the nearest integer).