Draw an entity-relationship diagram for the following situation: A gym has branches in three locations in a city. It has hired multiple trainers and maintains a daily schedule for them to adhere to.

In addition, there is a schedule for the general staff responsible for administrative work. Customer details such as the programs they have opted for and their gym timings need to be recorded in a system. The system should be able to manage overdue payments and the entry and exit times of the customers. The gym equipment also needs to be maintained and the inventory managed. List the necessary assumptions for this situation.