A relationship exists between litres and kilograms:

• 1 litre of water weighs, 1 kilogram.

A similar relationship exists between pounds and imperial gallons (IG):

• 1 IG of water, weighs 10 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 0.80 means that a volume of fuel weighs 80% of the equivalent volume of water.

For this question, 650 lb of fuel has a fuel volume of: 650 lb / 10 / 0.80 = 81 IG.

And, since 1 IG = 1.2 USG, then: 81 IG = 81 x 1.2 = 97 USG.

NOTES:
1. Fuel volume is measured in: Imperial Gallons (IG), US Gallons (USG) or litres (l or lt).
2. Fuel mass is measured in:, tonnes (t or ton), kilograms (kg) or pounds (lb).