Facts to twenty, then columns, then the stories that use them.
The answer key prints on a separate page.
Facts within twenty come first because everything after depends on them being automatic rather than worked out. A child counting on fingers through section 1 will be slow in section 2 for a reason that has nothing to do with columns — each column is a fact, and a page of columns is twenty of them.
The column section is where regrouping shows up, and where the reliable error is subtracting the smaller digit from the larger regardless of which is on top: 52 − 27 answered as 35 is not a slip, it is a rule being applied consistently in the wrong direction.
