If you have 222 g of radioisotope with a half-life of 5 days, how much isotope would remain after 15 days?

b. what percentage of a radioactive isotope would remain after four half-lives?

If the half life is 5 days, you would have to divide 222 by 2 3 times.

so 222/2 = 111g

111/2 = 55.5g

55.5/2 = 27.75g

after four half life the mass would be

27.75/2

13.875g

the percent would be

13.875/222

= 6.25%