Approximately 59.7739%.
Uranium has two naturally-occurring isotopes (note that despite the fact that both species are radioactive, they are the most abundant ones for having a relatively-long half-life.)
Assuming that [tex] x \; \%[/tex] of the uranium atoms in this sample are atoms of [tex] ^{235}\text{U} [/tex]; given the fact that other isotopes of uranium contribute to a negligible share of mass of the sample (less than [tex] 0.01\% [/tex] in naturally-occurring samples,) [tex] ^{238}\text{U} [/tex] would have contributed to [tex] 1-x\; \% [/tex] of all atoms in this sample.
Similar to the standard atomic mass for an element, the average atomic mass of this sample shall resembles the atomic mass weighted over all atoms in the collection. Therefore
[tex] \%^{235}\text{U Abundancy} \times m_a(^{235}\text{U}) + \%^{238}\text{U Abundancy} \times m_a(^{238}\text{U}) = a_r(\text{sample}) [/tex]
[tex] 235.043 \; x + 238.051 \; (1-x) = 236.253\\ x \approx 0.597739 = 59.7739 \; \% [/tex]
