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Hi; Here is the new solution.
| Gas Station | Price/Gal | Miles | Total Operating Cost | |
| Frank’s | 2.00 | 2.00 | $42.53 | |
| Joe’s | 1.90 | 6.50 | $46.15 | |
| Martha’s | 1.75 | 7.20 | $43.88 | |
| Bill’s | 1.70 | 8.00 | $43.81 | |
| Sue’s | 1.66 | 9.00 | $44.19 | |
| Ed’s | 1.92 | 11.00 | $52.22 | |
| Sally’s | 1.56 | 17.00 | $40.08 | |
| Tom’s | 1.38 | 18.00 | $41.21 | |
Assumptions: Filling in weekends back and forth operating cost.
To avoid Toing cost, filled 1 gallon gas at Sue's gag station and proceed to sally's and tom's.
Added fuel cost to go to home, that is additional cost would have saved if filled in closer gas station
While filling gas, at sally's how much gas will be filled is calculated this way ( 21-((15+15)-17)/15),
which is ((30-C8)/15) and multiplied by cost of gas per gallan at that gas station.
To avoid towing cost, one gallon of gas is added at Sue's gas station and added to the total cost.
Here is the formula;
| Gas Station | Price/Gal | Miles | Total Operating Cost |
| Frank’s | 2 | 2 | =(C2*0.5)*2+((21-(1-C2/15))*B2)+(C2/15)*B2 |
| Joe’s | 1.9 | 6.5 | =(C3*0.5)*2+((21-(1-C3/15))*B3)+(C3/15)*B3 |
| Martha’s | 1.75 | 7.2 | =(C4*0.5)*2+((21-(1-C4/15))*B4)+(C4/15)*B4 |
| Bill’s | 1.7 | 8 | =(C5*0.5)*2+((21-(1-C5/15))*B5)+(C5/15)*B5 |
| Sue’s | 1.66 | 9 | =(C6*0.5)*2+((21-(1-C6/15))*B6)+(C6/15)*B6 |
| Ed’s | 1.92 | 11 | =(C7*0.5)*2+((21-(1-C7/15))*B7)+(C7/15)*B7 |
| Sally’s | 1.56 | 17 | =(B6*1)+(C8*0.5*2)+((21-((30-C8)/15)*B8))+(C8/15)*B8 |
| Tom’s | 1.38 | 18 | =(B6*1)+(C9*0.5*2)+((21-((30-C9)/15)*B9))+(C9/15)*B9 |
This is my final answer and Tom's the cheapest.