MgO
[tex]M_{Mg} =24u\\M_{O} =16u\\\frac{M_{Mg}}{M_{O}} =\frac{24}{16} =\frac{3}{2} \\M_{Mg}:M_{O}=3:2[/tex]
M Mg = 24u
M O = 16u
M Mg : M O = 24:16 = 3:2
CaO
[tex]M_{Ca} =40u\\M_{O} =16u\\\frac{M_{Ca}}{M_{O}} =\frac{40}{16} =\frac{5}{2} \\M_{Ca}:M_{O}=5:2[/tex]
M Ca = 40u
M O = 16u
M Ca : M O = 40:16 = 5:2