The Starving Dead

It’s Halloween, and a horde of zombies starving for brains are attacking the city, but you are lucky enough to work in a fast-food restaurant where the main course is brains!! Be quick and serve sets of soda and brains to satisfy the hunger of these starving zombies monsters before they get to you!
But be careful! Some zombies are polite enough to return the trays, and if you let them fall the noise can further stressed that violent horde of zombies!
How long can you survive to the zombies?