Answer:
yes.
honestly, I didn't really do all the math.
3 in 18 minutes, a little better than 3 in 20 minutes. that translates to 9 in an hour, with 6 minutes left over.
3 hours of that (3×9) is the 27. with 6 minutes per hour left over, for a total of 18 minutes.
the teacher has time to grade the tests, get a cup of coffee, and take a few minutes to wonder why she didn't go into an easier line if work.