Last week, she used 15 gallons * 0.2 = <<15*0.2=3>>3 gallons less fuel than she used this week.
This means that she used 15 gallons – 3 gallons = <<15-3=12>>12 gallons of fuel last week.
In total, Mary used 15 gallons + 12 gallons = <<15+12=27>>27 gallons of fuel.
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