Sudoku Tutorial
Hidden Singles

Hidden single:  When a candidate is present only one time in a house, if that cell contains
more than one candidate, it is a HIDDEN SINGLE.

In row 5, candidate 3 is only present once (r5c1).  This eliminates the 7
and changes the cell to a naked single (3).

(In this example, the 3 is also a hidden single for box 4)


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