A relationship exists between litres and kilograms: one litre of water weighs one kilogram.
A similar relationship exists between pounds and imperial gallons (IG): one IG of water weighs ten pounds.

To convert from volumes to mass or mass to volumes, you need the Specific Gravity of the fuel.

The Specific Gravity (SG) shows the relationship between litres and kilograms or between pounds and imperial gallons.
Specific Gravity 0.81 means that a volume of fuel weighs 81% of the equivalent volume of water.

For this question, 865 lb of fuel has a fuel volume of: 865 lb / 10 / 0.81 = 106.77 IG, rounded 107 IG.