You can put this solution on YOUR website!
Before getting on the highway Jenna drove 60 miles, while getting 15 miles per gallon. That means that the amount of gas she used for that part of the trip, in gallons, is
(60 miles)/(15 miles/gallon) = 4 gallons
For the entire trip, she travelled 200 miles, getting an average of 25 miles per gallon. That means that the amount of gas used for the entire trip, in gallons, is
(200 miles)/(25 miles/gallon) = 8 gallons
The portion of the trip where Jenna drove on the highway was 140 miles long and must have consumed
8 gallons – 4 gallons = 4 gallons of gas.
The average fuel consumption for that part of the trip is
140 miles/4 gallons = 35 miles/gallon.
I see no need to make this problem any more complicated than a fourth grader (or even a fifth grader) would understand.