The following navigation utilizes arrow, enter, escape, and space bar key commands. Left and right arrows move through main tier links and expand / close menus in sub tiers. Up and Down arrows will open main tier menus and toggle through sub tier links. Enter and space open menus and escape closes them as well. Tab will move on to the next part of the site rather than go through menu items.
This video models addition and subtraction using representations for place value.