How many gallons are in 390 pounds?

If the question relates to gallons of water, then in the UK I gallon = 10 lbs. In the USA 1 gallon = 8.35 lbs approx.

In the UK 390lbs is the weight of 390/10 = 39 gallons of water.

In the USA 390lbs is the weight of 390/8.35 = 46.71 (approx) gallons of water.