Scanning through the rows and columns also helps familiarize yourself with the entire puzzle right away.
If the row contains 2 and 3, the column contains 8, and the square contains 4 and 7, then write 1, 5, 6, and 9 into the cell since they’re the only possible numbers.
Check the row, column, and square for the cells you just filled in and erase any other times the number you just wrote appears. That may help you find more single cells you can fill in. You may also find “hidden singles” in the puzzle. If you notice a number only appears once in the row, column, or square regardless of the other numbers in the cell, then you know that that digit has to go there.
If 3 and 9 are the only options for 2 cells in the same column, then erase 3 and 9 pencil marks from the other cells. You can also find “hidden pairs” if the pair of numbers are only in 2 cells within the same row, column, or square regardless of other numbers in the square. For example, one cell in a square has 3, 5, 7, and 9 and another cell in the same square has 3, 7, and 8. If 3 and 7 don’t appear anywhere else in the square, then they have to appear in those 2 cells.
For example, if the 4 empty cells could all contain a 5, then the 5 would have to appear in the top corner of one column and the bottom corner of another. You can erase every other 5 in the same rows and columns.
Since the number can never share the same row or column, you limit the possible locations you can place them.
You can even work on an entire row or column of 3 x 3 squares at a time to help you see which values are missing.
“Hard” or “extreme” puzzles are the toughest and give you the least numbers in the grid, so you’ll have to use logic a lot more to fill in the cells.
If the timer goes off before you’re able to finish, try giving yourself a few extra minutes next time.
