That is everybody's/everyone's problem should both be accepted
shouldn't it be 문제이다?
문제이다 -> 문제다 (for noun ending in vowel only: this loss of "이" is called 'particle elision' which happens in casual Korean language - not academic works).
Are you saying the 이 in 이다 is a particle?
Bad explanation. Apologies. What I meant is
문제다 is just a contracted form of 문제이다. "이" of the linking verb 이다(to be) is usually dropped after a vowel-ending noun.
can you use 모두 다 in place of 모두의 and still get the same meaning?
모두다의 , yes. 다 is just an added emphasis.
Note: without the possessive 의, 모두다 문제이다 = everyone/everything is a problem.
That's everyone's problem should be excepted. Come on Duolingo!!
Just report it. There are tons of solutions and no one could come up with every one of them.
It didn't accept "That problem is everyone's." Though the meaning in English between that and "That is everyone's problem." are identical.
I can't believe they don't accept that problem is everyone's... I was so confused