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?
Last week, she used 15 gallons * 0.2 = 
<gadget id="calculator">15*0.2</gadget>
<output>3</output>
3 gallons less fuel than she used this week.
This means that she used 15 gallons - 3 gallons = 
<gadget id="calculator">15-3</gadget>
<output>12</output>
12 gallons of fuel last week.
In total, Mary used 15 gallons + 12 gallons = 
<gadget id="calculator">15+12</gadget>
<output>27</output>
27 gallons of fuel.

<result>27</result>