That's not right. Zahlen not only mean quantity. Nummer is actually "digits with special meaning with the given orders". So you may say 18 is a number only when you're talking about the house/street/hotel room number in German. Nummer is not a definition for any digits in German. Zahlen is.
Duo wants to know if you know the German words for the numbers.
On this course, they are not interested if you also know the English symbols for the numbers in question. The fact that the English number symbols are the same as the German symbols doesn't mean you can avoid answering with the German words by using the symbols from English that you already know.
Don't bother reporting it.
Hello biertopf, greetings from the Rhineland! I think this question would be too difficult for most German native speakers. I would write "Die Neunzehn ist eine Zahl." The article makes sure that 19 is a noun in this sentence. So I question myself why "neunzehn" is not capitalized without an article.
Die Neunzehn ist eine Zahl. Achzehn und neunzehn sind Zahlen.
Is it the same with colours? Rot und grün sind Farben. Dieses Rot ist schön. (this kind of red is beautiful)
I had a picture before with 1,2,3 (maybe you all know it) where I couldn't comment but why "The numbers" are not "Nummern" but Zahlen and here I get Nummern as Numbers. Can it be that up to 9 are all "Zahlen" and afterwards are all "Nummern". We have this in romanian so I'm guessing it might be here the case too. Thanks in advance.
I miss this explanation in the other posts, so I'll try: Numbers from 0 to 9 are called Ziffern (Singular die Ziffer). Every Zahl is build with at least one Ziffer. Nummer is a function of Zahl, it determines a sequence, for example houses in the street have Hausnummern up from 1 to xyz (depends on the length of the street).
If I understand you right, it sounds like you had a multiple choice Choose-the-right-picture exercise where one of the pictures was labeled "Nummern"? Just because the option used "Nummern" doesn't mean "Zahlen" isn't also right. Duo only uses one translation at a time; it won't show you all possible translations, so you just have to do the exercise with whatever words it gives you.
Can it be that up to 9 are all "Zahlen" and afterwards are all "Nummern". We have this in romanian so I'm guessing it might be here the case too.
No, the difference is that "Nummer" is just a label (like a house address), whereas "Zahl" is a number with actual mathematical significance (like a counted number of objects). (A rule of thumb I've heard is that "Zahlen" can be meaningfully added together but "Nummern" can't. So 10 owls plus 20 owls is 30 owls, but adding house number 10 and house number 20 is just another random house that has nothing to do with the ones you added.)
In sentences like these where there's no context to dictate which kind of number it is, you should be able to use either one. I believe "Zahlen" is also accepted for this exercise, or at least others on this page have commented that it is probably a more appropriate answer.
Maybe the plural form of "Nummer" is introduced by this sentence?. If you want to know everything before using it, Duolingo is not the suitable system for you. Take a book to learn German. Read it through an then do the tasks and think about starting Duolingo again afterwards.
§ 58.6 in Duden says that numbers below one million must not be capitalized although they seem like nouns.
From the second link: "Aber dann kann man ja mit der Vierzehn zur Jugendherberge zurückfahren (aber: mit der Linie vierzehn). "mit der Linie vierzehn" itself is short for "mit der Linie Nummer vierzehn", "vierzehn" is a number here.
So numbers in the meaning of "Nummern" are not capitalized.