Just curious where you're getting that per gallon rate? Rental companies charge as much as $2 more than local going rate for a refill (maybe even more than that). Did the receip
20 hours in the car is a lot. Although if it was all stop and start driving (looking in a neighborhood), then perhaps it all fits.
It's exceedingly odd that she rented a car. 5 hours of idling per 15 hours of driving is truly a lot - a very high estimate. I think it's likely they paid $4 a gallon for that refill and went more like 600 miles. Still a lot. But I wouldn't be surprised if, with taxes, they paid more like $5 for a refill at Budget. According to the FTC, that's the typical rate ($5-6 per gallon) for Budget:
Budget Rent-A-Car Settles FTC Charges; Fuel Fees Levied on Customers Who Returned Rental Cars With a Full Tank
And the tank only holds 11-12 gallons, so how can they possibly have purchased more than that? That makes it closer to $9 a gallon (and they would have had to fill up while renting if they went more than 1 tank full - each tank gets them about 300 miles).