From the problem, we know that:
The half-life of the nuclide is 30 years.
The time that we're going to study are 120 years.
The first thing we need to do is to find the number of half-lives that has elapsed (n). To do this, we divide the time studied by the half life:
How to determine the original mass?
We know that the amount remaining (N) is = 4.1g and the number of half-lives (n) = 4. To find the original amount (No), what we do is to use the following equation:
The original mass of Cs-131 sample was 65.6 grams.
