Have you seen the guide by olimo? https://www.duolingo.com/comment/11536858
It's definitely correct too. If Duolingo doesn't accept it, I believe it should be reported.
Ok :D And I didn't even submit it as an answer, I was just wondering about the difference.
After к its always и. Why ы after н I dont know, but its easier to say for me. Also мужчины.
Try saying that word aloud both ways.
мужчины v мужчини
планы v плани
I hope it will help.
After н, both ы and и are possible.
When the stem (the part of the word to which endings are attached) ends in hard -н, the ending is ны: сло́н ‘elephant’ — слоны́ ‘elephants’.
When the stem ends in soft -нь, the ending is ни: конь ‘horse’ — ко́ни ‘horses’.
No, this would be «э́то ко́шки».