Comparing independent decays: simulations of the r-process suggest 1.35 nuclei of u were produced for every 1 nucleus of u in the supernova explosions that made the 235 238 particles that formed earth. the current relative abundances on earth are 0.7 and 99.3 percent, and the half lives are 7.0x10 y and 4.5x10 y, respectively how long ago were those uranium isotopes made? (yes, they're really from several different supernovae, but simplified for the problem. also, they were floating around for a while before earth was formed, so don't just write the age of earth.)