Since the half-life of 235U (7.13x10^8 years) is less than that of 238U (4.51x10^9 years), the isotopic abundance of 235U in natural uranium has been steadily decreasing since the earth was formed. How long ago was the isotopic abundance of 235U in natural uranium equal to 3.0 atom percent, the enrichment of uranium used in many nuclear power plants?