Any natural language (and Russian is no exception) sometimes confuses people accustomed to engineering thinking. If you are interested in this topic, please sit back. Go.
It all starts simply, with unique numbers: One, two, three ... eighteen, nineteen.
Dozens
Next comes the formation of tens, adding to the unique numeral ending:
Two - two (twenty), three - three (twenty), four - four (twenty)? Not. Forty! Why? Don't ask.
Five - five (ten). Here again a turn. Another unique ending for the formation of tens, but it seems the most logical - “ten”. Five tens - five (ten). It seems to sound good and even logical. And I was not the only one, apparently, I thought so, because further they go: six (ten), seven (ten), eight (ten) ... and here again someone distracted Uncle Fyodor and Sharik finished writing - ninety!
Why do dozens have different endings? Why do the second ten have unique names for numbers at all? Why then are not all numbers up to a hundred unique? How did the second dozen stand out so much?
Maybe someone has an answer to these questions other than "it happened historically", but I don't know it. It would be interesting to listen to.
But be that as it may, we will summarize and generalize what we have.
We have three groups
Unique numeral + ending denoting tens (twelve, ten)
- ,
.
, , (), ()... - (), (), (), (), ().
, . , , ! . .
+ (, )
- ,
19 () .
8 () . , , .
9 () .
, , ?
, . .
. , .
, "" - (), (), (), (), (). , "" - . . .
: (), (), (), (), (), (), (), (), ().
.
, , "" - (), (), (), (). , , , "" - . . .
, , . . , ? .
: (), (), (), (), (), (), (), (), ().
9 () .
2 () ("" "").
.
, , .
PS Just in case, I do not call for organizing revolutions or reforms. I just analyzed the current state of affairs through the eyes of an engineer and pondered how this problem could have been solved if the engineer had initially taken on its solution.
All the best!