x, y are the two elements, they are following law of multiple proportions. The ratios of x, y by weight are

Standard definition for law of multiple proportions: Different weights of an element that combine with a fixed weight of the other element bear a simple numerical ratio.

Ex: Carbon and Oxygen combine to form Carbon monoxide and carbon dioxide.

In Carbon monoxide, exactly 8g of oxygen is combined with 6g of carbon. Whereas in CO₂, 16g (exactly twice 8g) of Oxygen is combined with 6g of Carbon. Weights of Oxygen which combines with a fixed weight of carbon (6g) are in the ration of 1: 2.