There is a hidden pair 13 in box 3.

You can see 13 in row 1 and column 9, leaving only the 2 cells r23c7 (marked x) in box 3 for them. So they must be there, and you can eliminate the other candidates from the2 cells, and all 13's in the rest of column 7.

- Code: Select all
` +-------+-------+-------+`

| 3*7 1*| 2 . 6 | 5 - - |

| 8 6 4 | . . 5 | x 2 - |

| 9 2 5 | . . 4 | x 6 - |

+-------+-------+-------+

| 4 1 7 | 5 . . | 6 3 2 |

| 6 9 8 | . 2 . | . 5 . |

| 5 3 2 | 6 4 7 | 9 8 1*|

+-------+-------+-------+

| 2 5 . | 7 . . | . . 3*|

| 7 4 . | . 5 2 | . . 6 |

| 1 8 . | 4 . . | 2 7 5 |

+-------+-------+-------+

Yes, in this case it would have been sufficient to see, that 1 is restricted to the 2 cells, and can't be in r8c7.