Example #55

Strontium ⁹⁰₃₈Sr has a half-life of 28.5 yr. It is chemically similar to calcium, enters the body through the food chain, and collects in the bones. Consequently, ⁹⁰₃₈Sr is a particularly serious health hazard. How long (in years) will it take for 99.9900% of the ⁹⁰₃₈Sr released in a nuclear reactor accident to disappear?

Example #56

Charcoal from an ancient fire pit is found to have a carbon-14 content that is only 17.5% that of an equivalent sample of carbon form a living tree. What is the age of the fire pit?