Question

Does anyone know how to calculate exponential regression on a graphic calulator? I already entered values into my statistics utility but I'm still confued. My problem asks that if a substance has a life for 24 years, and a sample of 80 grams is being observed, how much will remain in 50 years? When will only 5 grams remain? I'm very confused on how to make my exponential model for this problem. Please help.