Give the step-by-step reasoning process and then the final answer. Lisa, Jack, and Tommy earned $60 from washing cars all week. However, half of the $60 was earned by Lisa. Tommy earned half of what Lisa earned. How much more money did Lisa earn than Tommy?

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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Lisa earned $60 * 1 / 2 = $30. Tommy earned $30 * 1 / 2 = $15. Lisa earned $30 - $15 = $15 more than Tommy.
The final answer: 15.