4. Iodine-131 is a radioactive isotope of iodine that is used to treat thyroid cancer. Because the thyroid gland uses iodine to make hormones, when patients take I-131, it travels directly to the site of their cancer, the thyroid. As the I-131 decays, the radiation it produces kills the cancer cells. The half-life of I-131 is 8.04 days. It undergoes decay. What percent of the I-131 will remain in a patient’s system after 40.2 days?