What is the cause of decrease in volume of gaseous mixture in the following problem? I am studying Gaseous State.

The problem says: A mixture of gas contains $\ce{N2}$ and $\ce{C2H2}$. $\pu{20 \\mL}$ of this mixture is added to $\pu{70\\mL}$ of $\ce{O2}$ and combustion is allowed to take place over mercury. After cooling the volume of gas was found to be $\pu{68.5\\mL}$. When the resultant gas mixture is passed through a $\ce{KOH}$ solution, the volume of residual gas is $\pu{38.5\\mL}$. Then the percentage composition of $\ce{N2}$ is:

In the solution to the question there is this following step which I am not getting:

:Volume of $\ce{CO2} = \pu{68.5\\mL} - \pu{38.5\\mL}$.         How come?
Also please clarify why nitrogen does not undergo combustion.

Please help.
I think the fact you are missing is that after combustion, water vapors will be given away and the volume lost during combustion will be equal to the amount of water lost. $\ce{KOH}$ will absorb $\ce{CO2}$ and the volume absorbed by $\ce{KOH}$ will be equal to volume of $\ce{CO2}$ in the solution. Thus, you will have two equations and you will be able to proceed with the solution.