I know that the reaction between bleach and hydrogen peroxide produces oxygen but using this apparatus and information [![enter image description here][1]][1] I need to state how to calculate the volume of oxygen released. 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 $20\ \mathrm{cm^3}$. I fail to understand this as I believe that the $20\ \mathrm{cm^3}$ of the hydrogen peroxide reacts with the bleach completely and even if not, will still nly produce oxygen and not any other gas, making the use of subtraction unnecessary and wrong for our answer. 


  [1]: https://i.sstatic.net/b2xOq.jpg