Total number of invoices = 30,800
Invoices produced by each computer = 30,800/2 = 15,400
Rates:
Computer 1 = 7,700/hr
Computer 2 = 6,800/hr
Time taken by computer 1 = 15,400/7,700 = 2 hrs
In 2 hrs, invoices produced by computer 2 = 2*6,800 = 13,600
Invoices remaining after the two hours when computer 1 finishes = 15,400 - 13,600 = 1,800 invoices.