Our local scrap merchant had to travel to the county recycling plant, 60 miles away.
On the way there he could only manage a steady 12mph, fully laden.
On the way back, he could manage a steady 16mph.
He placed 24 gallons of fuel in the tank ready for the round trip, thinking that this would be enough as the wagon managed to travel 8 miles per gallon. Unfortunately, a piece of metal pierced his fuel pipe at the start of the trip and the wagon started to lose 1 gallon of fuel per hour.
Luckily he only lost fuel when moving and he lost no fuel whilst unloading his scrap metal.
The trip was a total of 120 miles, which at 8 miles to the gallon, used 15 gallons.
The outward trip of 60 miles was at 12 mph, which took 5 hours. The return trip was at 16 mph, which took 3.75 hours. Which is a total journey time of 8.75 hours, which meant that 8.75 gallons were lost.
So, 15 gallons were used for the trip, 8.75 gallons were lost, which is a total of 23.75 gallons. So the 24 gallons was enough for the return trip.