If you start with 64g of a radioactive element how many half-lives would occur before 8g remain?

3 half-lives

Explanation:

The half-life is the time that it takes to a radioactive element to decay to half of its initial amount.

Let's suppose we start with 64 g of the radioactive element.

After 1 half-life, the mass of the element will be 32 g. After 2 half-lives, the mass of the element will be 16 g. After 3 half-lives, the mass of the element will be 8 g.