You are responsible for maintenance for a fleet of delivery trucks. Truck number 1517 traveled 551.7 miles and used 23.6 gallons of gas this week. Truck 1519 traveled 417 miles and used 19.8 gallons. Truck 1524 traveled 656 miles and used 25.7 gallons. Truck 1530 traveled 342 miles and used 15.1 gallons. What was the average gas mileage for these trucks for the week?