Mass fraction of Mg in MgxOy is 60%. Derive the compound formula.

Let’s designate: m – the molecular weight of the compound. From the condition, x and y are the number of atoms of the elements.
The relative atomic mass of magnesium is 24, oxygen is 16.
If magnesium in the substance is 60%, then the oxygen in the substance is 40%. Let’s compose the following expressions:
0.6 = 24x / m;

0.4 = 16y / m;

Let’s express the molecular weight:
m = 24x / 0.6 = 40x;

m = 16y / 0.4 = 40y;

Equating the values: 40x = 40y. This means that x = y = 1. The formula of the substance is MgO.

Answer: the formula of the compound is MgO.