Duo lists the correct translation as "Some of you..." - shouldn't that be "alguns de vocês sabem..."? With algum in the singular I think this is most naturally translated into English as "One of you knows..." but that was marked wrong.
Yes... that should not be wrong!
If "saber" takes the singular "sabe," then why is it "some of you know" rather than "one of you knows"?
because it was a literal translation English-Portuguese.