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%