Strontium-90 has a half-life of 28.5 years. How long (in years) will it take for 94.9900% of the strontium-90 released in a nuclear reactor accident to disappear?
123 years, to three significant figures
94.99% of a quantity gone means that 5.01% is left, or, fractionally, 0.0501 of it.
When one half-life has elapsed, 0.5 of it is left. The question is how many halvings leave us with 0.0501, i.e. find
This is a logarithmic problem:
For a half life of 28.5 years, this is (to three significant figures) 123 years.