A technique to date geological samples uses rubidium-87, a long-lived radioactive isotope of rubidium (t_½ = 4.8 × 10¹⁰ years). Rubidium-87 decays by β emission to strontium-87. If rubidium-87 is part of a rock or mineral, then strontium-87 will remain trapped within the crystalline structure of the rock. The age of the rock dates back to the time when the rock solidified. Chemical analysis of the rock gives the amounts of ⁸⁷Rb and ⁸⁷Sr. From these data, the fraction of ⁸⁷Rb that remains can be calculated. Suppose analysis of a stony meteorite determined that 1.8 mmol of ⁸⁷Rb and 1.6 mmol of ⁸⁷Sr (the portion of ⁸⁷Sr formed by decomposition of ⁸⁷Rb) were present. Estimate the age of the meteorite.