And because, actually, it is il numero uno, like it is always la persona. We have the same problem in French, people start to make confusion between the gender of the word with the holder, which are actually independent. And it is good using this way, if we do not know who is the number one, it would be confusing saying it.
I think you may have missed the point being made by Kathy above, Susie; while what you say makes sense in that it's good to see how language is used in various ways, I think what Kathy was saying was that in her particular case she was asked to translate from the English 'you are number one' in which case one would be perfectly correct to translate this in a female or male context. Although, I must admit, I find it strange that DL would ask someone to translate something which could clearly be interpreted in various ways like this without specifying gender first, as a clue.
"Il numero uno" is correct. What we have here is what looks like faulty gender agreement, because it can be regarded as a contraction of "tu sei la donna che è il numero uno".
Let's say you wanted to call someone a "dunno". That would have to be a "nonlosò". It would have to phrased as "tu sei un nonlosò" for a man and "tu sei una nonlosò" for a woman. It's not because of the inherent gender of the noun (after all, I've just invented it), but because it's clearly to be understood as "tu sei una donna che dice sempre «non lo so»".
So it is here. The woman isn't being described as merely a number (which would necessarily be "numero" and masculine) but as the entire phrase "numero uno".
With respect, you miss the point. There is nothing in the exercise which could lead the student to understand that a female is being addressed. Therefore it is perfectly fine to compose the reply as if a male was being addressed, as much as it is also correct to compose the reply as if it were addressed to a female. Both replies are equally correct, unless the gender of the person had been explicitly stated a priori.
"numero" is masculine, for sure! but in this sentence "Tu" is a girl just because of the "la" Tu sei la numero uno = you [girl] is the number one tu sei il numero uno = you [boy] is the number one In english it doesn't make any sense because "the" is used for both genders (I dnt know if I was clear enough) :(
Actually in french, as in italian, the gender of the word may be different to the person or the thing represented. But, today, politicians for "feminism", when they are talking about a lady, say for example "la numero" or another thing, in France. I guess it is the same thing there, and in this case, in a good language, to say "la numero" is wrong. Actually it is a modern error for an elementary grammar course.
Apparently not, unless you think the headline writers at La Repubblica are making elementary errors in Italian grammar: Serena Williams resta la numero uno e confessa: "Sarebbe bello avere un figlio"
When we are learning new languages, it would be helpful if DL give a rationale for the answer. How would I know to use LA here, but in another similar sentence when I opted to use LA it was marked wrong. What is the difference when I write "Tu sei la numero uno!" and "Io sono la numero uno!" (I am a woman, so I thought using the feminine article would be okay). So far I have not read a clear explanation.
I cannot see how "la numero uno can be right". Numero is a masculine noun and therefore must be preceded by il, not la. In the sentence "She sees her brother" it is incorrect to say "Lei vede la sua fratello" just because the subject is feminine. It has to be "il suo fratello", where the adjective and the article have to agree with the masculine noun fratello. That same rule of grammar must apply here.
sorry, should have been clearer. if it is referring to a feminine gendered word it is 'la numero uno'; if masculine 'il numero uno'. if you add any adjectives in they agree with the gender of the article (la mia numero uno, il mio numero uno). here is a page with some examples. https://context.reverso.net/traduzione/italiano-inglese/numero+uno
And here is a headline from La Repubblica which seems to confirm what you say: Serena Williams resta la numero uno e confessa: "Sarebbe bello avere un figlio"
NO, it's not. You don't learn anything when they tell you got it wrong when in fact you got it right. When they ask you to translate 'you are number one', there is NOTHING to suggest that this sentence refers to a female. This is just really, really irritating and reduces confidence in Duolingo. They should have de-bugged it by now.