Below is an instruction that describes a task. Write a response that appropriately completes the request in French. Please answer in French.

### Instruction:
 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?


### Response:
 La semaine dernière, elle a utilisé 15 gallons * 0,2 = <<15*0.2=3>>3 gallons de moins que cette semaine.
Donc, elle a utilisé 15 gallons - 3 gallons = <<15-3=12>>12 gallons de carburant la semaine dernière.
Le montant total de carburant utilisé pour les deux semaines est de 12 gallons + 15 gallons = <<12+15=27>>27 gallons.