16.71 this is because if you take the amount of fuel she started with, then subtract it by what she has left it gives you what she used. So. 18.41-1.7=16.71 and 16.71(what she used) plus what she still has 1.7 is 18.41. (16.71+1.7=18.41) so to summarize you just subtract what she has now by what she started with.