The isotopes U238 and U235 occur in nature in the mass ratio 140:1. It is assumed that initially they were found in equal mass. If half life (t½) of U238 = 4.5 × 109 and t½ of U235 = 5 × 108 year respectively, then the age of earth in years is (log 7 = 0.846; log 2 = 0.3)