\(\frac{ N _{ B }}{ N _{ A }}=\frac{ e ^{-\lambda_{2} t }}{ e ^{-\lambda_{1} t }}= e ^{-\lambda_{2} t } \cdot e ^{\lambda_{1} t }\)
\(e ^{1}= e ^{\left(\lambda_{1}-\lambda_{2}\right) t }\)
\(\left(\lambda_{1}-\lambda_{2}\right) t =1\)
\(t =\frac{1}{\lambda_{1}-\lambda_{2}}=\frac{1}{25 \lambda-16 \lambda}=\frac{1}{9 \lambda}\)