So, with 12 check outs and 4 stays over you will have to clean up 16 rooms ( I assume that the hotel has the habit of cleaning up the room even if people stay overnight)
so the time between 9 am and 3 pm is 4 hours, that is 4*60=240 minutes.
This means it's 240/16=120/8=60/40=15, 15 minutes per room.
About the first part of the sentence: I assume that you can start washing the dishes alrady at 6:30 (dishes you used for preparation,etc) - so you can clean up from 6:30 till 9:30 - that is three hours!
