A car travels 300 miles using 15 gallons of gasoline. What is the unit rate of miles per gallon? If the car needs to travel a total of 1200 miles, how many gallons of gasoline will it need? Also, if each gallon of gasoline costs $3, how much will it cost to travel 1200 miles? Let's think step by step and output the final answer within \boxed{}.
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