if in a question to verify law of  multiple  proportions

for eg

CO2 and CO

in  case of carbn dioxide % of oxygen is 72.7% and of   carbon 27.3%

in case of carbon monoxide % of oxygen is 57.14% and carbonis 42.8%

in this case  in order to verify law of definite proportions  i multiply percentages in each case to molecular masses of their respective compounds for eg.

72.7/100*44=32 and 27.3 /100*44=12

and 57.14/100*28=16   and 42.8 /100* 28 =12 so 32:16 =2:1