the time taken for the radioactivity of a specified isotope to fall to half its original value.
COUNT: after ONE After TWO After THREE After FOUR
640 divide by 2 Half-life half lives : half-lives: half-lives:
320 160 80 40
this shows that it takes FOUR half-lives for the activity to fall from 640 to 40.
so 2 HOURS represents FOUR half-lives -- so the half life is 2 hours divided by 4 = 30 MINUTES