We can calculate how long the decay by using the half-life equation. It is expressed as:
A = Ao e^-kt
where A is the amount left at t years, Ao is the initial concentration, and k is a constant.
From the half-life data, we can calculate for k.
1/2(Ao) = Ao e^-k(30)
k = 0.023
0.04Ao = Ao e^0.023(t)
t = 140 sec
