Let time be \(t\)
\(\lambda_1 \times e^{-\lambda_1 t}=\lambda_2 \times e^{-\lambda_2 t}\)
\(\lambda_1 e^{-\lambda_1 t}=\lambda_2 e^{-\lambda_2 t}\)
\(\frac{\lambda_1}{\lambda_2}=e^{\left(\lambda_1-\lambda_2\right) t}\)
\(\ln \frac{\lambda_1}{\lambda_2}=\left(\lambda_1-\lambda_2\right) t\)
\(\ln 1-\ln 2=\left(\lambda_1-\lambda_2\right) t\)
\(0.693=\left(\lambda_1-\lambda_2\right) t\)