Define the average life of a radioactive substance. 

Two species of radioactive atoms are mixed in equal number. The disintegration constant of the first species is $\lambda$ and of the second is $\lambda / 3$. After a long time the mixture will behave as a species with mean life of approximately

A radio-active material is reduced to $1 / 8$ of its original amount in $3$ days. If $8 \times 10^{-3}\,kg$ of the material is left after $5$ days. The initial amount of the material is $…….\,g$

Half life of radioactive element is $12.5\; Hour$ and its quantity is $256\; gm$. After how much time (in $Hours$) its quantity will remain $1 \;gm$

The decay constant $\lambda $ of the radioactive sample is the probability of decay of an atom in unit time, then