A 55-gallon drum can hold approximately 19.6-gallon cans. To calculate this, you can divide the volume of the 55-gallon drum by the volume of a 1-gallon can.
$$Number of 1-gallon cans = \frac{Volume of 55-gallon drum}{Volume of 1-gallon can}$$
$$Number of 1-gallon cans = \frac{55 \ gallons}{1 \ gallon}$$
$$Number of 1-gallon cans = 19.613 gallons$$
Since you can't have a partial can, the closest whole number is 19 cans.