Question

Suppose you have a cache of radium, which has a half-life of approximately 1590 years. how long would you have to wait for 1/5 of it to disappear?

Answer 1

You would have to wait 318 years for 1/5 (20%) of the Radium to disappear. 1590 is the point where half of the Radium would decay, so if you take 20% of that, it would equate out to 318 years. Radium has a long half life!