Question

A radioactive substance is decaying exponentially. if there are initially 15 grams of the substance and after 120 minutes there are 4 grams, what is the half-life in minutes of the substance?

Answer 1

4 g / 15g = .267 of original left

1/2^n = .267 where n = number of half-lives

n log -.301 = -.573

n = 1.90 number of half-lives occurring

120 / 1.90 = 63.2 min / half-live

Note: 2 half-lives = 126 min

After 126 min 1/4 or 3.75 g remain