"You think you are lucky?"
Translation:Bạn nghĩ bạn may mắn?
"Bạn nghĩ bạn may mắn không?" should be "Do you think you are lucky?". In my opinion, I think "Bạn có nghĩ bạn may mắn không?" is more natural.
"You think you are lucky !" would seem to be a better match for the translation. As others said, why no có or không??
And here's my answer: "Do you think..." is a straight yes/no question that needs a "có...không". "You think..."although technically a question is more like a declaration and should be as presented by Duolingo.