So how do we fill in this grid of number anyway? Here’s one technique:

0) First, for each blank cell on the grid, determine all the possible values the cell can have. This can be done by looking at the seed values in all other cells of the associated groups (row, column, and 3×3 square).

1) Whenever the value of a cell is known, that value can be eliminated from the list of possible values for all other cells of the associated groups.

2) Look for an unsolved cell that has just one possible value. That becomes the value of the cell. Then apply rule 1.

3) Look through all the unsolved cells of a group looking for a possible value that exists only in one cell. That value now becomes the value of that cell. Then apply rule 1.

Repeat steps 1, 2, and 3 until either all cells are solved or you reach an impasse. If the puzzle is not yet completely solved, then you need to use the advanced techniques.

With a bit of practice, for most puzzles, you don’t actually have to list out all possibilities for the unsolved cells. When starting to solve a puzzle, you can often easily spot several cells where the value is obvious. For some puzzles then, the challenge is to write down the solution in pen without jotting down any notes. But for many puzzles, at least some annotation is needed to keep track of possibilities.

Enjoy!