kvin

Numerals never take -j or -n, so it is kvin librojn with the endings on the noun alone and never kvinjn. Beyond that, kvin behaves like any other root and grows the usual family: kvina is fifth, kvine is fifthly, kvino is a group of five, kvinobla is fivefold and kvinono is a fifth part. Building ten out of it works by simple juxtaposition too, kvindek for fifty, written as one word.