You have a compound that is subject to decay. If you originally started with 80 g of the compound and you ended up with 5 grams 30 days later, what is the half-life of the compound?

Answer 1

I would do it this way.
ln(No/N) = kt
ln(80/5) = k*30
Solve for k, then
k = 0.693/t_1/2