When making restitution for a wrong, what did Leviticus require the guilty person to add to the full value?

Answer: A fifth

The offender repaid in full, added a fifth of the value, and brought an unblemished ram to the priest as a guilt offering.

“Regarding any holy thing he has harmed, he must make restitution by adding a fifth of its value to it and giving it to the priest, who will make atonement on his behalf with the ram as a guilt offering, and he will be forgiven.”— Leviticus 5:16 (BSB)