i am taking upgrading, and one of the main reasons i failed physics 30 to start with was due to half life. the book cant explain it good enough to me and either could my teacher. so i have a few questions that i would appreciate a starting point for. How much time elapses before 90% of the radioactivity of a sample of 72/33As dissapears as measured by its activity? the half life is 26h. A sample of radioactive material contains 10^15 atoms and has an activity of 6.00 x 10^11 Bq. What is the half life? these are just a couple of the many questions i am struggling with.