Have you begun to use the 100's chart with your class this year?
Remember to color code the return sweeps like this:
Why? I personally have difficulty tracking lines of text, and I know there are students that have the same problem. It's also hard to learn to visually sweep your eyes from 40 to 41.
A good independent activity is to have students color code the 100's chart that they use on the back cover of their Math Journal also.