Question

Iodine-125 has a half-life of about 60 days. How many milligrams of a 1000 mg sample will remain after 240 days?

Answer 1

240 days is 4 times the half-life, which means that a sample of 1000 mg will decay to

$\dfrac{1000}{2^4}=\dfrac{125}2=62.5\,\mathrm{mg}$

after 240 days.