Give the step-by-step reasoning process and then the final answer. Mary used 15 gallons of fuel this week. Last week she used 20% less. How much fuel did she use in total for the two weeks?
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Last week, she used 15 gallons * 0.2 = 3 gallons less fuel than she used this week. This means that she used 15 gallons – 3 gallons = 12 gallons of fuel last week. In total, Mary used 15 gallons + 12 gallons = 27 gallons of fuel.
The final answer: 27.
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