calculate the percent activity of the radioactive isotope iodine-131 remaining after 5 half-lives.

after 1^st half life 50% will decay away.

after 2^nd halflife only 25% of the original atom will remain

after another half-life only 12.5% of the original half-life will remain.

after 4^th half-life only  6.25% will remain

and after the 5^th half-life only the 3.125% will remain.

therefore after 5 halflifes only 3.125% of the original samle will remain