We can calculate how long will it take 90% to 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(3.3)
k = 0.21
.10Ao = Ao e^- 0.21(t)
t = 10.96 hrs
