"This question is very easy."
The problem could be easily be greater than the question. You could have trouble with question and still understand how to solve the problem- I translated elementary school math problem for a foreign student and basically their long-winded questions with calculate which with the diagrams is much easier.
Either is fine. On a test/exam/quiz, the words "problem" and "question" are interchangeable. My Japanese native friends and colleagues used 質問 and 問題 interchangeably in this context, too.
kantan - simple (kihonteki - simple), yasashii - easy (also yasashii - homonym - gentle/kind)
Its a good question actually, because it highlights yet another inconsistnlency. Another sentence has "easy test" in it, and if you tap on the word "easy", the popup help only has "kantan" in it. But in the word blocks, you only find "yasashii" to use. And tbh I don't even remember "kantan" being in N5, though that may have changed in recent years?
They used mondai to mean question instead of shitsumon. I have always used shitsumon for question and mondai for problem. But seeing it used like this here does make me wonder if mondai can be used like "a maths' problem" in English - meaning a maths' question.
I thought shitsumon means question. Now they're saying its mondai, even though the first quiz in this lesson implied it was shitsumon
The problem with this problem is that there isn't a second ん sound to use. So the only answer is このもんだいはとてもかんた です。 It should be このもんだいはとてもかんたんです。
Something that I've noticed only very recently on a few lessons is that Duolingo has not just been splitting words up into bits but it has also been taking the start of one word and attaching it to the end part of another. For instance, there may well have been a tile, not with a lone ん but instead something like んです or this もかん - the n from kantan added to desu or the mo from totemo added to the kan from kantan. It has almost caught me out a few times. This may have been the case for you. Duo is getting trickier! Now that you know that Duo is doing this though you can watch out for it and hopefully not get caught out next time.
I took a screenshot of what I had. I'd I could upload it here, I would. There was literally only One ん character in all of the tiles. When I went back to answer it the second time, after getting it Wrong the first time, there were was an extra tile with the ん on it and I could easily answer -finally.