prize9-7, Exercise 2: Half-life calculation

Working: one quarter remaining means two half-lives have passed (1 -> 1/2 -> 1/4). 2 x 100 million years = 200 million years.

Assumptions: the rock started with no daughter isotope (or the original amount is known), the system has stayed closed so no parent or daughter has been added or lost, and the half-life is accurately known.

Care: real measurements include uncertainties, so the age would be given as, for example, 200 +/- a few million years, and cross-checked with another isotope system.