Solving the 6x6 Grids of a Sheet
Open each grid with the cages that allow a single set of digits: two cells of sum 11 hold 5 and 6, three cells of sum 15 hold 4, 5 and 6. Next, look at a box of two by three that the cages cover except for one cell: that cell is the 21 of the box minus their sums. On a sheet of four, small candidates fit best in the top corner of each cell.
How the Three Levels Differ
On easy grids, four digits are already written and the cages are short, so the first moves come quickly. Medium grids give no digit, and every placement starts from a cage sum. Hard grids merge cells into cages as long as five, whose sums allow many combinations, so the work moves to the totals of rows, columns and boxes.
Who These Sheets Are For
These sheets suit solvers who have finished a few 4x4 killer sudoku grids and want longer chains of deduction before the 9x9, and classes that practice addition while they reason. A4 paper and a black-and-white printer are all they need.