A theory of nuclear astrophysics proposes that all the elements heavier than iron are formed in supernova explosions ending the lives of massive stars. Assume equal amounts of 235U and 238U were created at the time of the explosion and the present 235U/238U ratio on the Earth is 0.007 25. The half-lives of 235U and 238U are 7.04 108 yr and 4.47 109 yr, respectively. How long ago did the star(s) explode that released the elements that formed the Earth?