science

definition 

the time taken for the radioactivity of a specified isotope to fall to half its original value.

INITIAL

     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