nelisenkymmentä

Adverb

 * 1) about forty

Declension
The word nelisenkymmentä is not inflected. If one wants to say e.g. "in about forty" in Finnish, the easiest possibility is to use the adverb as determiner to the numeral :
 * noin neljässäkymmenessä tapauksessa sadasta = in about forty cases out of one hundred