Chapter 32 - Nuclear Physics and Nuclear Radiation - Problems and Conceptual Exercises - Page 1153: 32

$1621.5s$

We know that $\lambda=\frac{\ln 2}{T_{\frac{1}{2}}}$ $\implies \lambda=\frac{0.693}{122s}$ $\implies \lambda=5.68\times 10^{-3}/s$ Now $\lambda T=\ln 10^4$ This can be rearranged as: $T=\frac{\ln 10^4}{5.68\times 10^{-3}/s}$ $\implies T=1621.5s$