please answer part C correctly the expected answer is : $20$ meters away. I do not understand how this was obtained please clearly write out the formula and its relation to the rest of the problem.

We are standing $2$ m from a radioactive sample with an activity of $0\mathrm{.}2$ Ci $($where $1$ Curie

represents an activity of $3\mathrm{.}71010$ decays$/$second$).($For the first three parts, ignore absorp$-$

tion.$)$

$($a$)$ What would the intensity be at this distance? $($I$.$e$.,$ how many particles per m$2$ per

second?$).$

$($b$)$ How many clicks$/$sec would a Geiger counter pointed at the sample record, if the surface

area of the business$-$end of the counter is $5$ cm$2?$

$(****$ PART C $***$ what i need help w$/->$ c$)$ How far away would we need to move to reduce the intensity by a factor of $100?$

$($d$)$ We can also stay where we are and reduce the intensity by wearing some shielding

material like lead. If we use a shielding material that is $1$ mm thick, what would it$$s

linear attenuation coefficient $($and half$-$value layer thickness$)$ need to be$?$