Gay Lussac’s Law of Combining Volumes states that when gases react, they do so in volumes which bear a simple ratio to one another, and to the volume of the product(s) formed if gaseous, provided the temperature and pressure remain constant. 

  C\(_2\)H\(_{4(g)}\)  +  3O\(_{2(g)}\) → 2CO\(_{2(g)}\)  +  2H\(_2\)O\(_{(g)}\)

 1 mole  : 3 moles

Total volume required: 10 cm\(^3\)  50 cm\(^3\)

Reacted Volume:  10 cm\(^3\)  30 cm\(^3\)

Residual volume: 0  (50 - 30) = 20 cm\(^3\)