d
\(X_{1}=N_{0} e^{-\lambda_{1} t}\) ;  \(X_{2}=N_{0} e^{-\lambda_{2} t}\)

\(\frac{X_{1}}{X_{2}}=e^{-1}\) \(=e^{\left(-\lambda_{1}+\lambda_{2}\right) t}\) ; \({e^{ - 1}} = {e^{ - (\lambda 1 - \lambda 2)t}}\)

\(\therefore t=\left|\frac{1}{\lambda_{1}-\lambda_{2}}\right|=\frac{1}{(5 \lambda-\lambda)}=\frac{1}{4 \lambda}\)