Silpat Kura: You should have mentioned if you were close to my answer? There there are assumption and I tried to match most of them.

Assumption: 1) The car drives upto 15 miles the gas in the tank and assumed that the car gives 15 miles per gallon.
2) Total operating cost to fill in the whole tank as it said shows how much gas it will be filled.
3) Calculation is done according to the distance it is traveled to various gas stations.
4) Going back it will cost gas as well cost .50 per mile operating cost.
Did not add back the cost of gas going back to home.
Most constraints are trying to  considered to come with a realist value.

Now Are you close to any of us? What is your answer?? How did you do?
Should have written here rather than crappy answer.