you really should have a mass given but since you dont just choose a simple one like 40
three fourths of forty is 30
how long will it take for iodine-131 to decay from 40-30
it would take 8.1 days for it to decay from 40-20
so it would take half that time for it to decay till 30
so 8.1/2
it would take 4.05 days
(this will work with any amount)
