You received a shipment 20 days ago of 131i for treatment of hyperthyroidism. what fraction of the original shipment would you still have with a half-life of 8.040 days for i?

Explanation:

There are many formulas that can describe the exponential decay of a substance. For example, one of the formulas we could use for the quantity that still remains after a time t, given an original quantity and a half-life of is:

We want to calculate what fraction of the original shipment would still we have, that is,

This is why it is useful to use the formula already written, now we can just calculate:

, which means that around 17.83% of the original substance has not decayed yet.