The uranium ore mined today contains only $0.72\%$ of fissionable ${}^{235}U$,too little to make reactor fuel for thermal-neutron fission.For this reason,the mined ore must be enriched with ${}^{235}U$. Both ${}^{235}U(T_{1/2}=7.0\times 10^{8}y)$ and ${}^{238}U(T_{1/2}=4.5\times 10^{9}y)$ are radioactive. How far back in time would natural uranium ore have been a practical reactor fuel,with a ${}^{\frac{{}^{235}U}{{}^{238}U}}$ ratio of $3.0\%$?