A geological sample is found to have a Pb-206/U-238 mass ratio of 0.773/1.00. Assuming there was no Pb-206 present when the sample was formed, how old is it? The half-life of U-238 is 4.5 × 109 years.
3.98 × 109 years
1.4 × 1010 years
2.1 × 109 years
7.3 × 1011 years
2.4 × 1010 years