First off I just wanted to say something quickly. I am a Visual Arts major taking a Introductory Physics class for fun, (probably my favorite class that I have ever taken, also the most rewarding. I have learned so much that I wish I had taken it sooner so I could have pursued it more) My physics professor gave out a study guide for our final with questions and their respective answers. I have been able to complete every question flawlessly except for one. I have tried everything that I have been capable of to solve this problem but I can not seem to figure it out (even researched online, but all results went over my head). So here we go. 1. The problem statement, all variables and given/known data The isotope 90Sr has a half-life of 28 years. How many years will it take for 7/8 of a sample of this material to decay (so that 1/8 is left)? The solution to the problem listed in the answer key is 84 years. 2. Relevant equations This is where I have become confused, I really do not have notes for this type of problem, only for problems with a similar subject matter. The equation I have is [tex]\alpha[/tex]= (2.7x10^-11)n [I am not sure how I would use these though, I feel like they do not work for this problem, I have used it on others] 3. The attempt at a solution I have tried quite a few things. I assumed that 28 years being the half life, is half of its life. So I thought maybe the full length of decay would be 28*2 which would be 56. Then I took 7/8 of that number to find out how long it will take to have 1/8 of it left. My answer for that was 49 years. Is my train of thought correct in any way when it comes to this problem or am I missing something completely? I just need to be guided in the right direction so I can solve this myself when a similar problem comes along on the final.