Question: Hawkeye is driving to his aunt. He has to charge his battery for $3.5 per charge. If he charged his battery four times, and his battery charging budget was $20, how much money was he left with when he reached his aunt's place? Think carefully first, then make a decision:

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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If each battery charge costs $3.5, the total expense of 4 battery charges is 4 * 3.5 = $14. After his expense on battery charging, his balance was 20 - 14 = $6. So the answer is 6.