Nick starts with 20 milligrams of a radioactive substance. the amount of the substance decreases by 12 each week for a number of weeks, w . he writes the expression 20(12)w to find the amount of radioactive substance remaining after w weeks. autumn starts with 1 milligram of a radioactive substance. the amount of the substance decreases by 50% each week for a number of weeks, w . she writes the expression (1-0.5)w to find the amount of radioactive substance remaining after w weeks.