So, from what I am reading it looks like they are just asking what happens when the volume increases when mixed gases go to a different volume. Since I do not know enough about the prompt nor the mixed gases it is hard for me to answer this 100% right because the mix gases can change the pressures/gas laws if it was originally just oxygen. If absolutely nothing else changed, such as temperature or kinetic energy and it was the same exact gas from one volume to the next, I would say use Boyle's law P1V1=P2V2, where you have all the information in your question, but are looking for one key thing missing, here the new pressure of the mixed gases. (1)(1) = (P2)(3) ... now all you do is solve for the new Pressure ie P2. I hope that helps!