Дано

w(Fe) = 72.46 = 0.7246

Решение

FexOy

w(Fe) = xAr(Fe) / (xAr(Fe) + yAr(O))

0.7246 = 56x / (56x + 16y)

0.7246 (56x + 16y) = 56x

0.7246 (56x + 16y) = 56x

40.5776x + 11.5936y = 56x

11.5936y = 56x - 40.5776x

11.5936y = 15.4224x

y = 1.33x

3y = 4x

x : y  = 3 : 4

Ответ: Fe3O4