Two oxides of metal contain $36.4 \mathrm{and} 53.4$ percent of oxygen by mass respectively. If the formula of the first oxide is ${\mathrm{M}}_2\mathrm{O}$, then that of the second is

An element $\mathrm{X}$ has the following isotopic composition:

${}^{200}\mathrm{X}:90\raisebox{1ex}{$ \mathrm{o}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{o}$}\right.$ ,  ${}^{199}\mathrm{X}:8.0 \raisebox{1ex}{$\mathrm{o}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{o}$}\right.$ ,   ${}^{202}\mathrm{X}:2 \raisebox{1ex}{$\mathrm{o}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{o}$}\right.$

The weighted average atomic mass of the naturally occurring element $\mathrm{X}$ is close to :