How much is 20 percent of 25 gallons?

Assuming that you are referring to 25 kilograms of water (density = 1 kg.dm-3), this mass corresponds to 25 litres or 25000 mL. A UK gallon is eight pints or 8 x 568 mL = 4544 mL A simple division 25000/4544 = 5.50 gallons (2 d.p.) So five-and-a-half gallons of water weigh 25 kilograms.