Can you guide me through solving this problem: Arnold owns three cars. The first car averages 50 miles per gallon of gas. The second car averages 10 miles per gallon of gas. And the third car averages 15 miles per gallon of gas. He splits his 450-mile monthly driving mileage equally amongst his three cars.  If gas costs $2 per gallon, how much does he spend on gas each month?