Learn Finnish: Hänen osuutensa on viidesosa.

It is made by combining the ordinal number viides (fifth) with the word osa (part). This is the standard way to form most fractions in Finnish: you take the ordinal word and add osa.

Is there another way to say "a fifth" in Finnish?

Yes! Finnish also has special fraction words that often end in -nnes. A fifth can also be called viidennes. Both viidesosa and viidennes are perfectly correct and widely used.

Why is there an -nsa at the end of osuutensa?

It is the third-person possessive suffix. In Finnish, when you use a possessive pronoun like hänen (his/her), you must also attach the matching possessive suffix to the noun. For the third person, this suffix is -nsa or -nsä depending on vowel harmony.

What is the dictionary form of osuutensa, and why does it have a t in it?

The dictionary form is osuus (share or portion). Nouns ending in -uus or -yys change their stem to -uute- or -yyte- before adding most endings. So, osuus changes to the stem osuute-, and then the possessive suffix -nsa is added.