Radium-221 has a half-life of 30 sec. how long will it take for 96% of a sample to decay?

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

Is a logical series of steps

That is a lot of moles that you have