That's not how I read it. I read it as: Now : Mary is 24 = 2y, Ann is x Past: Mary is x, Ann is 12 = y Moreover, we have x + delta = 24, and 12 +

That's not how I read it. I read it as: Now : Mary is 24 = 2y, Ann is x Past: Mary is x, Ann is 12 = y Moreover, we have x + delta = 24, and 12 + delta = x (they get older at same rate), so delta = 6 and x = 18.