This example is to demonstrate how to apply the Hidden Single technique in a row.
In row R7 of this puzzle, there are four empty cells, which are (R7,C3), (R7,C4), (R7,C5), and (R7,C9). The digit 5 is not in this row yet. We want to decide which of these four empty cells should be filled with the digit 5. The Hidden Single technique is to look for the cells that are possible to accommodate the digit 5 in the row. If there is only one such cell, then that cell must be the digit 5.
As shown in Figure 2, the cell (R8,C1) in block B7 is 5. Since each block can only contain the digit 5 once, the cell (R7,C3) in the same block cannot be 5. This cell is eliminated as a possible position for the digit 5. Now three empty cells (R7,C4), (R7,C5), and (R7,C9) remain for the digit 5 in row R7.
As shown in Figure 3, the cell (R2,C5) in column C5 is 5. Since each column can only contain the digit 5 once, the cell (R7,C5) in the same column cannot be 5. It is eliminated as a possible position for the digit 5. Now two empty cells (R7,C4) and (R7,C9) remain for the digit 5 in row R7.
As shown in Figure 4, the cell (R4,C9) in column C9 is 5. Since each column can only contain the digit 5 once, the cell (R7,C9) in the same column cannot be 5, and it is eliminated as a possible position for the digit 5. Now only one empty cell (R7,C4) remains for the digit 5 in row R7.
The cell (R7,C4) is located in row R7, column C4, and block B8. No digit 5 appears in the same row, the same column, or the same 3x3 block. So the cell (R7,C4) can be a possible position for the digit 5 (Figure 5).
As we can see in Figure 5, the only possible position for the digit 5 in row R7 is the cell (R7,C4). Since each row must contain the digit 5 once, we can conclude that the cell (R7,C4), as the only possible position for the digit 5 in row R7, must be 5. The cell (R7,C4) should be filled with the digit 5, as shown in Figure 6 above.