You get paid for 27.5 hours. If you worked 27 hours jn 3 days and have holiday pay for the next two days, you are not getting overtime. It sucks, but that's the way it is.
Overtime is heavily scrutinized for PT sups and you need to have some justification especially if it's outside of peak. You are guaranteed 27.5 hours a week, not necessarily 5.5 hours a day.
RP