b
\(I\)                                                    \(II\)

\({\rm{M}}\,\,\,\,\,\,\,\,\,{\rm{:}}\,\,\,\,\,\,\,\,\,\,\,\,{\rm{O}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\rm{M}}\,\,\,\,\,\,\,\,\,\,\,{\rm{:}}\,\,\,\,\,\,\,\,\,\,\,\,{\rm{O}}\)

\(\frac{{{\rm{50}}}}{{\rm{x}}}\,\,\,\,\,\,\,\,:\,\,\,\,\,\,\,\,\,\,\,\frac{{50}}{{16}} = 1:1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{{40}}{x}\,\,\,\,\,\,\,\,\,:\,\,\,\,\,\,\,\,\,\,\,\,\frac{{60}}{{16}}\)

\(x = 16\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,2\,\,\,\,\,\,\,\,\,\,:\,\,\,\,\,\,\,\,\,\,\,\,\,\,3\,\,\,\,\,\,\,\,\,\,\,\,\,{M_2}{O_3}\,\,\)