The half-life of iodine-131 is 8.10 days. how long will it take for three-fourths of a sample of iodine-131 to decay?

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)