"They are girls and they have a book."
Translation:Ele sunt fete și ele au o carte.
I agree with you. For an English speaker, if this was put into two sentences it would make much more sense "Ele sunt fete." "Ei au o carte." It's like two different thoughts combined into one. Hence why it's confusing for them to be marked wrong when the sentence first starts off as they being referred to girls and then having the sentence be continued in the masculine form of 'ei.'
Wait... I am now in so much confusion... I answered "Ele sunt fete si ei au o carte." Firstly I was about to ask what the difference between Ele and ei is. I then see comments of it claiming ei is for masculine but the sentence was about two girls. Later on I look up and see that another translation of it is "Ele sunt fete si ele au o carte." Wow. S.O.S save our skin.
My guess is that the original sentence is in Romanian, with "ele" in each part of the sentence. That is translated as "they".
When that sentence is proposed in the opposite direction, here comes the problem, because English doesn't have distinct plural masculine and feminine pronouns. Are those two "they" the same group of people or two distinct groups? Is the first groupe feminine, masculine or mixed? What about the second group? All of this makes for four possible translations, with "ele" or "ei" possible translations for each "they".
I think the best way to translate that sentence to Romanian is to not put the subject pronoun. That way you keep the ambiguity from the English sentence.