Naturally occurring copper is assigned atomic mass equal to 63.546 u. If it consists of two isotopes ${ }^{63}\mathrm{Cu}$ u (atomic mass = 62.930 u) and ${ }^{65}\mathrm{Cu}$ (atomic mass = 64.928 u), then mass percentage of isotope ${ }^{65}\mathrm{Cu}$ in the naturally occurring copper is