Lithium occurs in two isotopes, namely, ${ }^7\mathrm{Li}$ (atomic mass: $7.00 \mathrm{amu}$ ) and ${ }^6\mathrm{Li}$ ( atomic mass: $6.00 \mathrm{amu}$). If there exists 7.4% of ${}^6\text{Li }$in naturally  occurring lithium, then its atomic mass will be