Step by step example of solving a level 5 (very hard) sudoku puzzle
Step 1
Let's look at the box in the center of the sudoku grid (where DEF cross MNO). We can't put a 7 in column N because there's a 7 on cell CN, and we can't put a 7 in cell DO because there's a 7 on cell GO. Therefore, we left with one option only: cell FM.