The ‘Last Possible Number’ technique is an effective strategy for beginners in Sudoku. In this method we examine the existing numbers in the puzzle and find the missing number in the 3×3 block, horizontal row, and vertical column.
For example, as shown in image 1, we need to find the missing number in the question-marked cell. Focus on the area with a white background and check out all the numbers presented in the 3×3 block as well as the shared row and column. Follow the below steps:
1. Write numbers 1, 2, 3, 4, 5, 6, 7, 8, 9
2. Now cross all the numbers already present in the 3×3 box: 1, 2, 3, 4, 5, 6, 7, 8, 9
3. Again cross all the numbers already present in the row: 1, 2, 3, 4, 5, 6, 7, 8, 9
4. Again cross all the numbers already present in the column: 1, 2, 3, 4, 5, 6, 7, 8, 9
What you are likely to discover is that only one number remains which is the number 5. Since each number can only appear once per block, row, and column, then it means that 5 can be the only possible figure for this cell. By using this method of elimination we can confidently put 5 into our question-marked cell as shown in image 2.
The ‘Last Possible Number’ technique makes Sudoku easier by eliminating numbers that have already been written using this method. Thus, it can hold on to the rule that no number should appear twice within any 3×3 block, column and row.
The ‘Last Possible Number’ together with the ‘Last Remaining Cell’ and ‘Last Free Cell’ are core strategies for solving Sudoku. In other words, getting good in the above three techniques will provide a good foundation for you to be ready for more complicated puzzles.
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