Can it be used:
1. To show what grade you are in? In Swedish we can say "I go in the two" (as in second grade (regular school/high school) "Jas odam vo dvojkata"
Why еден, but единица?
It is един/единица in Bulgarian, that is why he's asking about our еден/единица "inconsistency". The Macedonian-Bulgarian language "issue".I suppose that's because the i was short in (late Proto-Slavic) *edinu and was therefore turned into ь (yor, a so-called semi-vowel), which was later vocalised to a "full" vowel when in strong position (basically, when a vowel is necessary for pronunciation) or simply dropped when in weak position (*edьnъ > eden). In the latter word the i was long and therefore remained i.
In BCMS we also have jedan (a is the universal semi-vowel reflex in our language) with the genitive jednoga (the yor was in weak position here) and jedinica, jedinka, jedinstvo etc. on the other hand.
It is един/единица in Bulgarian, that is why he's asking about our еден/единица "inconsistency". The Macedonian-Bulgarian language "issue".
The bottom line is: we'd use "peticu" if we feel that "5" is just a label; "peti" if it's primarily an ordinal (floor, grade) and "pet" if we sense that it's a quantity. But perceptions may vary depending on speaker and situation and maybe even on the language (though I doubt that South Slavic languages would differ much here).