3 and 4 are subscripts. Please explain the method.

Given:

Two metallic oxides contain 27.6% and 30% oxygen respectively.

Ratio of metal and oxygen in first oxide, M₃O₄ = 72.4 : 27.6

Ratio of metal and oxygen in second oxide = 70 : 30

Let molecular mass of metal = M

Therefore, the percentage by weight of the metal in the oxide

Now,

Moles of metal in second oxide = 70 / 56 = 1.25

Moles of oxygen in second oxide = 30 / 16 = 1.875

Ration of moles of metal and oxygen = 1.25 : 1.875

   = 1 : 1.5

   = 2 : 3

Hence, formula of the second oxide = M₂O₃