Question

Starting with 195 grams of a radioactive isotope, how much will be left after 3 half-lives?

Answer 1

After one half-life:  $\frac{1}{2}\times 195=97.5$ grams.

After two half-lives:  $\frac{1}{2} \times 97.5=48.75$ grams.

After three half-lives: $\frac{1}{2} \times 48.75=24.375$ grams.