Rubidium-87 decays into strontium-87 by emission, with a half-life of years. A sample of rock contains 8.23 mg of rubidium-87 and 0.47 mg of strontium-87. Calculate the age of the rock to two significant figures.
Uranium‑238 decays into lead‑206 via a series of relatively short-lived nuclides. The half-life of uranium‑238 is years. A sample of uranium ore contains 9.22 mg of uranium‑238 and 2.84 mg of lead‑206. Calculate the age of the ore.
Isotopes such as are believed to have been present in the solar system since its formation. The half-life of is years and the age of Earth is years. Calculate, to two significant figures, the age of Earth when only 0.000001% of the original remained.