An equilibrium ore mixture has atom ratio for $235$U$/231$Pa of $2\mathrm{.}33$x$10\mathrm{⁴}.$ What is the half$-$life of $235$U from the data, given that the half$-$life of $231$Pa is $3\mathrm{.}28$x$10\mathrm{⁴}$ years.