In Sudoku, How Do You Figure Which Number Is Correct?

As you know, a number can't repeat in the same row or column. Also, a number can't repeat in the smaller squares within the puzzle.

Take a smaller square and see what numbers are already filled in, start taking the missing numbers and see where they might fit in the open spaces. You should end up with some opens that have 1 or 2 choices. If there is only 1 choice, then fill it in. That will eliminate that number from several other opens in the row or column and that should begin the completion of the puzzle. Some of the more difficult puzzles, you have to look at the entire row or column and find the number that can only fit in one space.

While playing sudoko we ensure that the same number doesn't exist in the same coloumn or row.