Define the average life of a radioactive substance.

(N/A) The average life (or mean life) of a radioactive substance is defined as the ratio of the total life time of all the radioactive nuclei to the total number of radioactive nuclei present initially.
Mathematically, it is the reciprocal of the decay constant $(\lambda)$.
If $N_0$ is the initial number of nuclei, the average life $(\tau)$ is given by:
$\tau = \frac{1}{\lambda}$
It is also related to the half-life $(T_{1/2})$ by the relation:
$\tau = \frac{T_{1/2}}{0.693} \approx 1.44 \times T_{1/2}$