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 = 10 g
Ao = 40 g
k = 0.00043
An = Aoe^-kt
10 = 40e^-0.00043(t)
t = 3223.94 years