1. The problem statement, all variables and given/known data The Earth is about 4.5 billion years old. If 235U is 0.65% abundant today, how abundant was it when the Earth formed? Note, in this case abundancy is defined as the ratio of Uranium 235 to Uranium 238 2. Relevant equations R=N(lambda) N=N0e^-lambda(t) Half Life = ln(2)/lambda 3. The attempt at a solution I am really unsure how to do this problem. I tried reworking the above equations but I was not able to get the correct answer. I need a little help getting started.