A 0.5 g sample of radioactive iodine-131 has a half-life of 8.0 days. After 40 days, how much is left? and What is the half-life of Po-214 if after 820 seconds, a 1.0 g sample decays to 0.03125 g? I want to understand how to do these questions, I'm in precalc with precalc knowledge.