A 9-microgram sample of a radioactive isotope decays to 7.61 micrograms in 7 min. What is the half-life of the radioactive isotope, in minutes? Use the exponential decay equation A = Aol where A is the amount of a radioactive material present after time t, k is the half-life of the radioactive t/k ' material, and Ag is the original amount of radioactive substance. (Round your answer to two decimal places.) min