My ds is wrapping up 2a right now. To be honest I gave him only the 1a and 1b tests. I looked through the 2a book and knew to start him with this one. Looking at the 2b book, he will need many of the skills learned in 2a to be successful. That being said, I think my ds will breeze through 2b pretty quick. From a quick glance it may be easier then 2a if that makes any sense.