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:
A0 = 18
An = 18/2 = 9
k = 0.125
9 = 18 e^-0.125t
0.5 = e^-0.125t
t = 5.55 days
