Strontium-90 is a radioactive waste product from nuclear reactors. It has a half-life of 29 years. How many years will it take for a quantity of strontium-90 to decay to of its original mass?

It will take 29 years to decay to its original mass; another 29 years to ; another 29 years to ; and another 29 years to :

29 + 29 + 29 + 29 = 116 years

Your Turn You have 180 g of a radioactive substance. It has a half-life of 265 years. After 1,325 years, what mass remains?