# A radioactive substance decays at a rate of 30% per year. Presently there are 400 grams of the substance. How do you find the amount left after 8 years?

At the end of $8$ year amount left
$= 400 {\left(1 - 0.3\right)}^{8}$
$= 400 {\left(0.7\right)}^{8}$
$= 400 \left(0.0576\right)$ grams
$= 23.04$ grams