For this problem, we use the formula for
radioactive decay which is expressed as follows:
An = Aoe^-kt
where An is the amount left after time t, Ao is
the initial amount and k is a constant.
We calculate as follows:
An = Aoe^-kt
0.5 = e^-k(4.5)
k = 0.1540
An = Aoe^-kt
An = 16e^-0.1540(18)
An = 1.00 mg
Hope this answers the question. Have a nice day.
