It takes 5 fifths to make a gallon.
It takes 10 fifths to make 2 gallons.
It takes 15 fifths to make 3 gallons.
It takes 20 fifths to make 4 gallons.
It takes 25 fifths to make 5 gallons.
A fifth in math terms is one out of five equal parts.
The fifth in math terms can also be written as 20% or 0.2.
But for every fifth in math terms you have 1 gallon.