Question

Americium-241 is used in smoke detectors. It has a first order rate constant for radioactive decay of k = 1.6x10-3 yr -1. By contrast, iodine-125, which is used to test for thyroid functioning, has a rate constant for radioactive decay of k = 0.011 day -1.1. How much of a 1.00 mg sample of americium remains after 4 days?2. How much of a 1.00 mg sample of iodine remains after 4 days?I know the answer I just don't know how to get to the answer, please explain. Thank You