Since the half life of U-235 (7.13E8 yrs.) is less than the half life of U-238 (4.51E9 yrs.) the isotopic abundance of U-235 has been decreasing relative to U-238 since the earth was formed about 4.5 billion years ago. Fission products have been found which indicate that natural reactors have existed in Africa (read Sec 4.9). This would likely have required the enrichment to be about 3.0 a/o. Currently U-235 is only 0.72 a/o or 0.711 w/o of natural U. Estimate how many years ago the natural enrichment of Uranium was 3.0 a/o. Note that a rigorous calculation would require accounting for the mass difference between U-235 and U238, and for the decay of U-238