Explanation:
Average atomic mass
\({\rm{ = }}\frac{\begin{array}{l}{\rm{\% }}\,\,{\rm{of}}\,\,{\rm{I}}\,\,{\rm{isotope}}\,\,{\rm{ \times }}\,\,{\rm{its}}\,\,{\rm{atomic}}\,\,{\rm{mass}}\,\,{\rm{ + }}\\{\rm{\% }}\,\,{\rm{of}}\,\,{\rm{II}}\,\,{\rm{isotope}}\,\,{\rm{ \times }}\,\,{\rm{its}}\,\,{\rm{atomic}}\,\,{\rm{mass}}\end{array}}{{{\rm{100}}}}\)
\({\rm{ = }}\frac{{{\rm{75}}{\rm{.53 \times 34}}{\rm{.969 + 24}}{\rm{.47 \times 36}}{\rm{.96}}}}{{{\rm{100}}}}\)
\({\rm{ = 35}}{\rm{.5}}{\mkern 1mu} {\mkern 1mu} {\rm{amu}}\)