1 gallon minus 1 quart, 1 pint, and 1 ounce.
The smallest unit is the ounce. We need to know the number of ounces there are in 1 gallon, 1 quart, and 1 pint
1 gallon = 128 ounces
1 quart = 32 ounces
1 pint = 16 ounces
128 oz - 32 oz - 16 oz - 1 oz = 79 oz
There are 79 ounces left from the initial 1 gallon.
79 oz * 1gal/128 oz = 79 gal/128 = 0.617 gallon