Stone Age Surfboards is a small manufacturer of two types of popular high-tide surfboards, the Rockwell and the Limestone models. The manufacturing process consists of two departments: fabrication and finishing. The fabrication department has 15 skilled workers, each of whom works 5 hours per day. The finishing department has 5 workers, who also work a 5-hour shift. Each pair of Rockwell surfboards requires 3 labor hours in the fabrication department and 1.5 labor hours in finishing. The Limestone model requires 4.5 labor-hours in fabrication and 2 labor-hours in finishing. The company operates 6 days a week. It makes a per-unit profit of $60 on the Rockwell model and $75 on the Limestone model. Approximately 4.1 Rockwell models and 8.4 Limestone models are produced per day. What is the total number of hours used for fabrication?