What fraction of a sample of 6832ge, whose half-life is about 9 months, will remain after 2.9 yr?

The half-life of 9 months is 0.75 years. 2.0 years is 2.0/0.75 = 2.67 half-lives. Each half-life represents a reduction in the amount remaining by a factor of two, so: A (t) / A (0) = 2^ (-t/h) where A (t) = amount at time t h = half-life in some unit t = elapsed time in the same unit A (t) / A (0) = 2^ (-2.67) = 0.157 15.7% of the original amount will remain after 2.0 years. This is pretty easy one to solve. I was happy doing it.