Half life of radioactive element depends upon

Starting with a sample of pure ${}^{66}Cu,\frac{7}{8}$ of it decays into $Zn$ in $15\, minutes$. The corresponding half life is..........$minutes$

Carbon dating is best suited for determining the age of fossils if their age in years is of the order of

A radioactive material has a half life of $10$ days. What fraction of the material would remain after $30$ days

A radioactive nucleus $A$ with a half life $T$, decays into a nucleus $B.$ At $t = 0$, there is no nucleus $B$. At sometime $t$, the ratio of the number of $B$ to that of $A$ is $0.3$. Then, $t$ is given by

$\beta$- rays emitted by a radioactive material are