Be sure to answer all parts.

The half$-$life of ${?}^{27}Mg$ is $9\mathrm{.}50$min.

$($a$)$ Initially there were $5\mathrm{.}20{10}^{12}{?}^{27}Mg$ nuclei present. How many ${?}^{27}Mg$ nuclei are left $28\mathrm{.}0$min later?

$$ clei

Enter your answer in scientific notation.

$($b$)$ Calculate the ${?}^{27}Mg$ activities $($in Ci$)$ at $t=0$ and $t=28\mathrm{.}0$min.

$t=0$

$$

$x$

$t=28\mathrm{.}0$min

$$