Radioactive decay can be described by the following equation in a = in ao- kt where ao is the original amount of the substance, a is the amount of the substance remaining after time t, and k is a constant that is characteristic of the substance. minutes for the radioactive isotope chromium-56, k is 1.17 x 10 if the original amount of chromium-56 in a sample is 34.5 mg, how much time is needed for the amount of chromium- 56 that remains to fall to 20.3 mg?