By 14 1 4 each week for a number of weeks, w . he writes the expression 20(14) 20 ( 1 4 ) w to find the amount of radioactive substance remaining after w weeks. autumn starts with 5 5 milligrams of a radioactive substance. the amount of the substance decreases by 50% 50 % each week for a number of weeks, w . she writes the expression (1−0.5) ( 1 - 0 . 5 ) w to find the amount of radioactive substance remaining after w weeks. use the drop-down menus to explain what each part of nick's and autumn's expressions mean.