Sue and her sister buy a $2,100 car. They agree to split the cost based on the percentage of days use. Sue's sister will drive the car 4 days a week and Sue will get the rest of the days. How much does Sue have to pay? Give me reasons, before answering the question

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
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Sue will drive 7 - 4 = 3 days a week. So she will pay 2100 * 3 / 7 = $900. So the final answer is 900.