The revolving door question is flawed...
So, the first two people enter at 0 seconds. At 5 seconds those two people are trapped between entrance and exit and two more people can then enter. However at 10 seconds the first two people are now at the exit so they leave and two more people enter the entrance.
This means that when done in reality there would never be more than 4 people in the revolving door at any given time (as you would have to purposely miss the exit to ever get to the third segment).
Therefore the answer should be 480. The question should be 'What's the maximum number of people that can enter Or leave the building in 30 minutes' for the answer to be 720.