My daughter can add like there is no tomorrow, but subtraction seems to be a different story :/. She can do fine if it's a problem like 19-8, but not 18-9. She automatically flips the 9 to the top so to her the problem is still 19-8 and she'd answer 11. I have tried just about everything I can think of including doing it on a marker board so it's larger with stickers, I've tried manipulatives, I've tried number flash cards so we can break the problem apart more. Any advice?