The average (mean) life at a radio nuclide which decays by parallel path is :

$A\stackrel{{\lambda }_1}{⟶}B\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }{\lambda }_{\mathit{1}}=1.8\times {10}^{\mathit{-}\mathit{2}}{\mathit{sec}}^{\mathit{-}\mathit{1}}$

$2A\stackrel{{\lambda }_2}{⟶}C\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }{\lambda }_{\mathit{2}}={10}^{\mathit{-}\mathit{3}}{\mathit{sec}}^{\mathit{-}\mathit{1}}$

We know about the

$\lambda ={\lambda }_1+{\lambda }_2$

 then 

$=20\times {10}^{-3}$

$t_{1/2}=\frac{1}{20\times {10}^{-3}}=50\sec$

The average (mean) life at a radio nuclide which decays by parallel path is  50 sec