After five half-lives, the percentage of original radioactive atoms left is ...... $ \%$

The disintegration rate of a certain radioactive sample at any instant is $4250$ disintegrations per minute. $10$ minutes later,the rate becomes $2250$ disintegrations per minute. The approximate decay constant is $......... \min^{-1}$.

In a sample of radioactive material,what percentage of the initial number of active nuclei will decay during one mean life?

The half-life of a radioactive substance is $20 \ min$. The time taken for the decay to increase from $20\%$ to $80\%$ is ........ $min$.

If the half-life of an element is $69.3 \text{ hours}$,what percentage of it will decay between the $10^{\text{th}}$ and $11^{\text{th}}$ hours? (Initial activity $= 50 \mu Ci$)

The half-life of $^{215}At$ is $100 \mu s$. The time taken for the radioactivity of a sample of $^{215}At$ to decay to $\frac{1}{16}$ of its initial value is .........$\mu s$.