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

In a radioactive element,the fraction of the initial amount remaining after its mean lifetime is:

The half-life of a radioactive substance is $20 \ min$. The time interval between $20\%$ decay and $80\%$ decay is ......... $min$.

$A$ radioactive sample decays by $10\%$ in one month. What percentage of the sample will decay in four months (in $\%$)?

The half-life period of radium is $1600$ years. Its average life time will be ....... years.

In an old rock,the ratio of uranium to lead nuclei is $1:1$. The half-life of uranium is $4.5 \times 10^9$ years. If the rock initially contained only uranium nuclei,how old is the rock?