An ergonomist suggests replacing two mail clerks, each of whom sorts 3,000 pieces/h and gets paid $10

Question:

An ergonomist suggests replacing two mail clerks, each of whom sorts 3,000 pieces/h and gets paid $10 per hour, with a machine vision system that can sort 6,000 pieces/h but costs $1 per hour to maintain and $50,000 to purchase. How many hours would it take to pay off the system?