$\begin{aligned} & N=N_0 e^{-\lambda t} \\ & \frac{N_B}{N_A}=\frac{e^{-\lambda_2 t}}{e^{-\lambda_1 t}}=e^{-\lambda_2 t} \cdot e^{\lambda_1 t} \\ & e^1=e^{\left(\lambda_1-\lambda_2\right) t} \\ & \left(\lambda_1-\lambda_2\right) t=1 \\ & t=\frac{1}{\lambda_1-\lambda_2}=\frac{1}{25 \lambda-16 \lambda}=\frac{1}{9 \lambda}\end{aligned}$