This is not a unique concept, flipping pennies to simulate Radioactivity, there are several if not dozens of these write ups. This is simply my particular write-up used successfully for my classes. If you don't already have a version of this lab, perhaps this can be of use. The critical piece is graphing the shape of the radioactive decay common to most radioactive elements. The graph allows one to estimate time frames that are not exact multiples of the half life. For example, tritium has a half life of 12.3 years, it is unclear (barring a scary equation) to determine what percentage of the original equation is left, Once a graph is created, a student is simply able to read the graph point.