How to calculate the volume of oxygen in a reaction between hydrogen peroxide and bleach using this apparatus

Question

I know that the reaction between bleach and hydrogen peroxide produces oxygen but using this apparatus and information, I need to state how to calculate the volume of oxygen released:

At the start of the experiment, the measuring cylinder is completely filled with water. Hydrogen peroxide solution $(\pu{20 cm3})$ is added to the flask using the syringe. When the bubling stops, the volume of gas collected in the measuring cylinder is recorded.
How would the volume of oxygen given off be calculated?

From my initial thoughts, I thought that the volume of oxygen will simply be the volume of the gas collected, however, the answer is said to be the total volume of gas collected subtract $\pu{20 cm^3}$. I fail to understand this as I believe that the $\pu{20 cm^3}$ of the hydrogen peroxide reacts with the bleach completely and even if not, will still only produce oxygen and not any other gas, making the use of subtraction unnecessary and wrong for our answer.

Answer 1

This is how it works in my head:

If instead of peroxide in the syringe, you just had $\pu{20cm^{3}}$ of air, then injecting the air will cause a reading of $\pu{20cm^{3}}$ to happen. The water gets pushed out of the way.

Injecting peroxide would do the same thing (approximately). After the reaction, there is still saltwater and leftover reactants taking up space. The oxygen produced will take up additional space (and push the water away) because it is a gas and it wants to expand.