So, after the mechanic worked on it, it only does 7 gallons for 350miles, how many gallons will it be at that rate, for 340 miles then?

well, each gallon is 4 bucks, for 6.8 gallons that'd be 6.8*4.
we already know that before the mechanic worked on it, it was doing 8gallons for the same 340 miles, and at 4 bucks a gallon that'd be 8*4.
how much is her savings?