Cs has a half$-$life of $30\mathrm{.}8$s$.($a$)$ If we have $8\mathrm{.}8$g initially, how many Cs nuclei are present?

$($b$)$ How many are present $2\mathrm{.}0$min later? $($c$)$ What is the activity at this time? $($d$)$ After how much time will the activity drop to less than about $1$ per second?