A radioactive substance decays with a decay constant k. At time t = 0, the amount of the substance is 2 mg. At t = 5 days the amount of the substance is 1.5 mg. Find the value of k, in–1 . Find a formula for the half-life of a radioactive material as a function of k. The half-life is the amount of time for a quantity to fall to half of its initial value. Compute the half-life of the radioactive substance described in this problem.