When it is written וחמישיותיו (fifth part)¹⁶³, it is composed of singular and plural. And just like it, עצתך (your counsels), זנותיכם (your strayings), תהלתיך (your praises), etc. This is because an abstract noun normally comes in singular, unless for example a man praises time and time again (plural תהלתיך) or a man advises time and time again (plural עצתך). And moreover, with respect to וחמישיותיו when one only adds one fifth, it will be in singular. And when adding a fifth of a fifth if one admits to owing the fifth, it will be plural. And this is according to the opinion of the sages of blessed memory, that one adds a fifth of a fifth, but not according to Rabbi Abraham ben Meir Ibn Ezra, whose opinion is that one pays two fifths.

Leviticus 5:24 says, או מכל אשר ישבע עליו לשקר ושלם אתו בראשו וחמשתיו יסף עליו לאשר הוא לו יתננו ביום אשמתו (or any thing about which he has sworn falsely, he shall even restore it in full, and shall add the fifth part more thereto; unto him to whom it appertains shall he give it, in the day of his being guilty).

footnotes: ¹⁶³ Leviticus 5:24.