Dustin has some money saved for a car he is going to buy. The car costs a total of $5,000 and he will put $300 every week into his bank account to help pay for the car. He will have enough money to pay for the car in 15 weeks. If Dustin was able to save $500 a week instead of $300 a week, how many fewer weeks would it take him to save enough money for the car?