After 5 half-lives, what percentage of the original number of parent atoms is left?  _______ (a number)

A half-life is the time it takes for half of the atoms in a sample to decay. After one half-life, 50% of the original atoms remain.

Here's how you calculate the remaining percentage after 5 half-lives:

1. After the first half-life, 50% of the original atoms remain.
2. After the second half-life, 50% of the remaining 50% (or 25%) of the original atoms remain.
3. After the third half-life, 50% of the remaining 25% (or 12.5%) of the original atoms remain.
4. After the fourth half-life, 50% of the remaining 12.5% (or 6.25%) of the original atoms remain.
5. After the fifth half-life, 50% of the remaining 6.25% (or 3.125%) of the original atoms remain.

So, after 5 half-lives, approximately 3.125% of the original number of parent atoms is left.