Traditional Jewish sources would translate that as 25%. It does say one-fifth, but it's a fifth in the sense that you pay 125%, and 25% is one fifth of what you pay. There have been shifts in a lot of mathematical concepts over time and that's just one example. For instance, at some points in history people did use fractions, but favored using one in the numerator - 3/5 would have been expressed as 1/2 plus 1/10.
My goal was to find counter-examples to the phrase "The notion of 2.5 goats didn’t make any sense to the people of those days. They would have been totally confused by the menu at Rotisserie Romados which offers 1/4 of a chicken."
