Iodine-125 has a half-life of about 60 days.

how many milligrams of a 500 mg sample will remain after 300 days?

1. 0.6 mg
2. 7.8125 mg
3. 15.625 mg
4. 31.25 mg

We know that Iodine - 125 has a half life of 60 days.
300 : 60 = 5 . It means that it was halved 6 times.
Therefore: 500 mg * 0.5^5 =
= 500 mg * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 =
= 500 mg * 0.03125 = 15.625 mg
3. 15.625 mg.

Think of it as five or above give it a shove. change the 5 or above to the right to a zero and add one to the number on the left.