An invoice audit requires processing 600 invoices. Technician A processes 30 invoices per hour, Technician B processes 36 invoices per hour, Technician C processes 20 invoices per hour. For the first 1 hour, only Technician A works. For the next 3 hours, Technician A and Technician B work. For the next 1 hour, Technician B and Technician C work. After that, Technician C works until all invoices are processed. What is the total time, in hours, from when processing starts?