$3.$ The linear attenuation coefficient of $3\mathrm{.}8$ cm thick absorber is $0\mathrm{.}139/$cm$.$ The absorber is placed into a radiation field where the count rate is $1200$ cps$.($a$)$ What is the attenuated count rate? $($b$)$ If another absorber$,$ comprised of the same material, reduces the count rate to $33\%,$ how thick is this absorber$?$

$4.$ The intensity drops by $86\%$ across an absorber$.$ The $($HVL$)$t of the absorber is $20$ g$/$cm$2.($a$)$ How thick $($in mass units$)$ is the absorber$?($b$)$ What is the mass attenuation coefficient of the absorber$$s material? $($c$)$ What is the mean$-$free$-$path?