Half-life is defined as the amount of time that it takes for radioactive substance to loose half its radioactivity. If a substance has a half life of 58 years and starts with 500 g radioactive, how much remains radioactive after 30 years?
You can work out a yearly decay-factor, which is just a growth factor (known from exponential functions in maths or economy), but now it's below 1.
We know that
After 30 years, activity will be:
On your calculator you can set this up like:
0.5 ^ (30 : 58) x 500 = 349