Question
Natural uranium is 0.7200% $^{235}\textrm{U}$ and 99.27% $^{238}\textrm{U}$ . What were the percentages of $^{235}\textrm{U}$ and $^{238}\textrm{U}$ in natural uranium when Earth formed $4.5\times 10^{9}$ years ago?
$^{235}\textrm{U}_0 = 23\%$
$^{238}\textrm{U}_0 = 77 \%$

Note: at 0:10 I mention the percentage of Uranium-238 as 99.2748%, but it's corrected in the closed captions (and in the written text) as 99.2745%.

Solution Video