The half-life of iodine -131 is 16 days. If you start with 100 gram sample, how many half-lives have past when you have less than a gram left?

Answer 1

2ⁿ = No/N.
No = 100
N = 1. Solve for n.
I get n - 6.64 half lives which means you need 7 half lives to get to less than 1 g. Six half lives leaves 1.56 g.