Not that I would have thought of this on the spot, but ... The numerator and denominator represent different things. The numerator is "how many things", and th

Not that I would have thought of this on the spot, but ... The numerator and denominator represent different things. The numerator is "how many things", and the bottom is "of how many spaces". Asking what fraction of the table is girls is dangerous, because the answer is (1 girl) / (1 girl + 2 boys). Still a stretch, rephrasing as "what fraction of seats are filled by girls" might have helped show the idea that the number of spaces is fixed.