This law states that “If two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other, are in the ratio of simple whole numbers or in simple multiple ratios”.
The law says that if there are two elements which combine together to form two different compounds, then if the mass of one particular element is fixed then the masses of the other element which combine with this fixed mass, are in simple whole number ratio or a multiple of it.
Carbon and oxygen combine to form carbon monoxide (Co) and Carbon dioxide (Co2).
Here the mass of C is fixed as 12 a.m.u. then the masses of 0 and 0 2 are 16 and 32. The ratio between them is 16:32 or 1: 2
The different masses of oxygen are 16 and 32 which combine with the fixed mass of carbon which is 12.