So much to love about Montessori but no other developer does it as well as these guys. Intuitive, thorough and an easy, beautiful format. So many ways for my 6 year old to practice adding and subtracting large numbers! The reward system doesn't out do the learning games so it's not a distraction which is a plus in my book. Having an opportunity to practice with manipulatives really solidifies any new concepts that come with adding and subtracting large numbers.