(Answer: Q = Q0 e0.139t ) b) Use your model to predict, to the nearest year, the time it takes one third of a sample of cobalt-50 to decay. (Answer: 3 years) Problem 3. Uranium-235 is used as fuel for some nuclear reactors. It has a half-life of 710 million years. How long will it take 10 grams of uranium-235 to decay to 1 gram?