Could be tricky if they didn't say just what weight they are quoting. If the use the weight stamped onto the tank itself, a number called WC, that is based on water weight, not propane, so the correct division is 51/8.3 and thus 6.15 gallons.
WC is the weight of the water needed to fill the tank 100%. But then you have to remember that a propane tank is only filled 80% and that propane weighs a lot less per gallon than water. A short cut formula is that the tank can hold propane equal to 42% of the WC value. A standard "20 lb" portable LP tank used on BBQ grills has a WC value of about 48.
But if they are quoting the actual weight of the propane in the tank, the numbers the others calculated for you are correct.
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