$A$ radioactive sample has a half-life of $5$ years. The percentage of the fraction decayed in $10$ years will be (in $\%$)

The half-life of radium is about $1600$ years. Of $100 \, g$ of radium existing now, $25 \, g$ will remain unchanged after .......... $years$.

Calculate the time (in $minutes$) interval between $33\%$ decay and $67\%$ decay if the half-life of a substance is $20\, minutes$.

$A$ sample of radioactive material $A$,which has an activity of $10\, mCi$ $(1\, Ci = 3.7 \times 10^{10}\, \text{decays/s})$,has twice the number of nuclei as another sample of different radioactive material $B$,which has an activity of $20\, mCi$. The correct choices for half-lives of $A$ and $B$ would then be respectively:

The half-life of a radioactive substance is $20 \text{ minutes}$. In $........ \text{ minutes}$ time,the activity of the substance drops to $\left(\frac{1}{16}\right)^{th}$ of its initial value.

$A$ radio-isotope has a half-life of $5$ years. The fraction of the atoms of this material that would decay in $15$ years will be