"The number of female students increased."
Translation:Kız öğrenci sayısı arttı.
I had the same question. Especially since there is another sentence in this unit that translates "the number of colors" as "renklerin sayısı." I think that "kız öğrenci sayısı" is a compound noun, whereas "renklerin sayısı" is a true possessive. But why can't there be a possessive in this sentence (or a compound noun in the other one)?