Weights of two different elements which are combining with a fixed weight of a third element, are also the weights with which they combine with one another or their multiples or sub multiples. This is the

According to law of multiple proportion and "law of reciprocal proportions".

Law of reciprocal proportions: If two different elements combine separately with the same weight of a third element. The ration of the masses in which they do so are either the same or multiple of the mass ratio in which hey combine.

${\mathrm{CO}}_2:12:32=3.8$

${\mathrm{H}}_2\mathrm{O}:2:16=1:8$

${\mathrm{CH}}_4:12:4=3:1$