The half-life period of a radioactive material is $15 \ minutes$. What $\%$ of radioactivity of that material will remain after $45 \ minutes$?

$A$ first-order nuclear reaction is half-completed in $45 \ minutes$. How long does it take for $99.9\%$ of the reaction to be completed in $hours$?

The order of a radioactive disintegration reaction is:

Gadolinium-$153$,which is used to detect osteoporosis (porous bones),has a half-life of $242$ days. Which value is closest to the percentage of $Gd-153$ left in a patient's system after $2$ years? .......... $\%$

If $n_t$ number of radioatoms are present at time $t$, the following expression will be a constant:

$A$ radioactive element has a half-life of $1 \text{ day}$. After $3 \text{ days}$,the amount of the element left will be: