if 1g of a drug whose half-life is 9 hours is administered intravenously, how much of the drug is left after 3 hours? after 5 hours? after 10 hours?
after t hours, the fraction left is
(1/2)^(t/9)
So, after 3 hours, there is (1/2)^(3/9) = .7937
of the original amount left.