Developing number sense is something I would have expected all my students to have done. Unfortunately, that is not the case. One activity I like to do to develop this number sense is something I learned in a conference.
1. Copy base 10 frames with different amounts filled with different amounts onto transparency film. (Yes it must be on transparency film.)
2. Put 3-4 frames up for students to see. This should only be up for 3-5 seconds.
3. Remove the frames and have students guess how many spaces are filled in (the total number - like 33, 22, etc.)
4. Put the frames back up. Place the frames on top of each other to show groups of 10's. (This is why it is important to copy on transparency film.)