A filler vowel is needed with certain consonant clusters. 100 is "stu" and "st" requires it, whereas "dwustu" (200), "trzystu" (300), "czterystu" (400) don't. I

A filler vowel is needed with certain consonant clusters. 100 is "stu" and "st" requires it, whereas "dwustu" (200), "trzystu" (300), "czterystu" (400) don't. I don't remember the list. "mn" is another combo that needs the vowel: "ze mnóstwa".