**A patient is administrated 20 mg of iodine-131. How much of this isotope will remain in the body after 40 days if the half-life for iodine -131 is 8 days?**

**Answer:**

0.625 mg.

Just divide the number by 2 for every 8 days.

So it’s 20 -> 10 in first 8 days 10->5 on the 16th day, 5->2.5, 2.5->1.25, then 1.25->0.625 mg.