What's the difference of Ста, Сти, Сот?
To make the pronunciations smooth?
Can you give examples of these words in a context?
I'm really sorry, but
I have forgotten what I was commenting on...
I'll organize my Ukranian memory, and
reply to you as soon as I can.
Сти does not exist. Сот/сті do not exist as separate words for numbers (сімсот - seven hundred, двісті - two hundred). Ста can be the genitive case of сто.
I really appreciate your cool explanations (including other threads)!
" do not exist as separate words"
means that the words are put in the end of a sentence,
or in the central?
It means that they have the meaning of "hundred" only as parts of other words: сімсот - seven hundred, двісті - two hundred, сотка (colloquial) - one hundred square meters.
does not accept "221 dollars"
The answer is correct only in American English.
Missing an "and" before 21
In US English, "and" in that position is optional (and is sometimes considered wrong). Unfortunately, the course authors appear to have experience only of US English.