Twenty two-digit problems a sheet, addition and subtraction mixed, each with one number replaced by a box: 34 + ___ = 61, or ___ - 18 = 25. The blank moves between the two operands, never the answer - a sheet that hid the answer would just be an ordinary worksheet.
At two digits the missing number is no longer something to count to, so it forces the actual relationship: to find the box in 34 + ___ = 61 you subtract, and to find it in ___ - 18 = 25 you add. Working out which operation undoes the one on the page is the skill these build.
An equation with one value replaced by a blank - 34 + ___ = 61. Solving it means running the arithmetic backwards, which is a different skill from working an ordinary problem left to right.
One of the two operands, chosen so the sheet alternates. The answer is never the blank; that would just be the standard worksheet.
Second and third grade, once two-digit addition and subtraction with regrouping are steady and the inverse relationship is being made explicit.
Yes. Turn on Include Answer Pages and each equation prints complete on the key, with the missing number filled in.