An excavating company charges $200 an hour for the use of a backhoe and $500 an hour for the use of a bulldozer. (Part of an hour counts as a full hour.) The company employs one operator for 40 hours per week to operate the machinery. If the company wants to bring in at least $18,200 each week from equipment rental, how many hours per week can it schedule the operator to use a backhoe?