MAY #4

It takes our janitor 15 minutes to clean a classroom. If there are extra shoes, pencils, erasers, glue sticks, pieces of paper, etc. on the floor, you can add another five minutes to that time. Last Wednesday night it took our janitor exacty two and a half hours to clean the eight classrooms on our floor. Within that time he had to make a trip out to the dumpster which took him a total of ten minutes.

How many classrooms was he able to clean in fifteen minutes?