Answer: $57,600
Step-by-step explanation:
Accounting works by the Accrual basis. What this means is that transactions are recorded in the period they occur not in the period they are paid for.
In the above, Lee Inc. billed its customers $57,600 for services performed during the year. This is a service company and since their revenue comes from billings, this is their revenue for the year.
It does not matter that some of the services have not been paid for, it matters that the services were performed during the year.