“And he shall add its fifth” — so that principal and fifth equal five parts; the fifth is external (one-fourth of principal).
In the sugya: The “fifth” is calculated as an external addition: on four units of principal one adds a fifth unit, producing a total of five. It is not an internal fifth of the principal itself.
Type of Learning
Diyuk of “chamishito” — the fifth is external (one-fourth of principal).
What Is Learned
On four perutot of principal one adds a fifth perutah so the total is five. The Torah’s language requires that the principal plus its fifth equal five parts; the surcharge is therefore one-fourth of the original amount.