Volume of products - stoichiometry
Here's a breakdown: First, you identify that ethyne is the limiting reagent and would be consumed completely. Second, you correctly calculated the volume of oxygen left that is $450\ \mathrm{cm^3}$. $20\ \mathrm{cm^3}$ of ethyne will completely react to produce $40\ \mathrm{cm^3}$ of carbon dioxide.

Thus, the total gas volume when the reaction reaches completion is $490\ \mathrm{cm^3}$.