Column subtraction with borrowing, constrained so no difference ever goes negative — print, practice, mark from the key.
Borrowing is the first place subtraction gets genuinely hard, and the only fix is volume with feedback. These sheets keep the difficulty honest — operands span the full two-digit range, so borrows appear on some lines and not others.
The separate answer-key page means whoever marks the sheet is not redoing the arithmetic — and the student cannot peek mid-sheet.