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.

$2C+O_2\to 2CO$

$2\times 12g32g$

$6g16g$

$C+O_2\to C{O}_2$

$12g32g$

$6g16g$

In Carbon monoxide, exactly 8g of oxygen is combined with 6g of carbon.  Where as in  $C{O}_2$ 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  $1:2$.