Question 1033113
The formula of the compound is {{{As[4]}}}{{{O[6]}}} , meaning that a molecule or formula unit comprises {{{4}}} atoms of {{{As}}} , and {{{6}}} atoms of {{{O}}} .
The mass of the {{{4}}} atoms of {{{As}}} ,
each with a mass of {{{75}}} atomic mass units, adds up to
{{{4*75=300}}} .
The mass of the {{{6}}} atoms of {{{O}}} ,
each with a mass of {{{16}}} atomic mass units, adds up to
{{{6*16=96}}} .
The total mass of the molecule is {{{300+96=396}}} atomic mass units,
and out of that total, the mass of the arsenic is
{{{300}}} atomic mass units.
As a fraction, that is
{{{300/396=about0.76=about76/100=about76%}}} (rounding to whole numbers, or 2 significant figures, as a chemistry teacher might say).