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

### 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 semana pasada, usó 15 galones * 0.2 = <<15*0.2=3>>3 galones menos de combustible que esta semana.
Entonces, usó 15 galones - 3 galones = <<15-3=12>>12 galones de combustible la semana pasada.
La cantidad total de combustible utilizado en las dos semanas es de 12 galones + 15 galones = <<12+15=27>>27 galones.