1. The problem statement, all variables and given/known data The uranium present in earth today is 99.28% for U-238 and 0.72% for U-235. The half-life of U-238 is 4.51E9 years and 7.0E8 years for U-235. How long ago was this uranium 50% U-238 and 50% U-235? 2. Relevant equations A=Aoe-kt 3. The attempt at a solution This is driving me nuts. I first thought of doing them both separately: Let A1 be U-238 and A2 be U-235 A1=99.28 A2=0.72 k1= ln2/4.51E9=1.53E-10 A1=Ao, 1e-k1t 99.28=50e-1.53E-10t ln(99.28/50)=-1.53E-10t t=-4.4E9 years = 4E9 years k2= ln2/7.0E8=9.9E-10 A2=Ao, 2e-k2t 0.72=50e-9.9E-10t ln(0.72/50)=-9.9E-10t t=4.3E9 years = 4E9 years But then I tried solving them where Ao should be equal for them. But somehow that doesn't make sense unless Ao is the 50%??? Anyways, here's the other way I tried: Equation 1: 99.28=Aoe-1.53E-10t Equation 2: 0.72=Aoe-9.9E-10t Rearrange Equation 2: Ao=0.72/e-9.9E-10t Plug Equation 2 into Equation 1: 99.28=(0.72e-1.53E-10t)/(e-9.9E-10t) 99.28/0.72 = (e-1.53E-10t)/(e-9.9E-10t) 137.9 = (e-1.53E-10t)/(e-9.9E-10t) ln(137.9) = -1.53E-10t + 9.9E-10t ln(137.9) = 8.36E-10t t = 5.9E9 years Arrrghh! This is so frustrating! I don't even know how to check which is the right answer. Can someone explain to me which method is correct and why? Am I putting the numbers in the correct A or Ao place??