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? Think carefully first, then make a decision:
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. So, the answer is 27.