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Got a hint from Draw/Play, but what's the logic behind it?

 
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gf1701



Joined: 27 May 2006
Posts: 4

PostPosted: Sat May 27, 2006 7:13 pm    Post subject: Got a hint from Draw/Play, but what's the logic behind it? Reply with quote

A friend sent me a puzzle from a NY Post Sudoku book, and we both got stuck at the same place, which I've shown below. I used the Daily Sudoku Draw/Play feature to give me a hint and it filled in row 6 column 9 with an 8:

+-------+-------+-------+
| . . 1 | 4 3 . | . . . |
| . . 4 | . 7 8 | 6 . 1 |
| . . 7 | . 5 1 | 9 . . |
+-------+-------+-------+
| 4 8 6 | 7 9 5 | 1 2 3 |
| 7 1 3 | 8 2 4 | 5 6 9 |
| 9 5 2 | . 1 . | . 7 ? |
+-------+-------+-------+
| . . 5 | . 4 . | . 1 . |
| 3 . 9 | 1 8 2 | . . . |
| 1 . 8 | 5 6 7 | 3 9 . |
+-------+-------+-------+

I can't figure out how I could have determined that. Can someone explain it to me? Here is the grid with the possibilities I had:

--------------------------------------------------------------------
|8652 962 1 | 4 3 96 | 72 85 752 |
| 52 932 4 | 92 7 8 | 6 53 1 |
| 862 632 7 | 62 5 1 | 9 843 42 |
|-----------------------------------------------------------------
| 4 8 6 | 7 9 5 | 1 2 3 |
| 7 1 3 | 8 2 4 | 5 6 9 |
| 9 5 2 | 63 1 63 | 84 7 84 |
------------------------------------------------------------------
| 62 762 5 | 93 4 93 | 872 1 8762 |
| 3 764 9 | 1 8 2 | 74 54 7654 |
| 1 42 8 | 5 6 7 | 3 9 42 |
--------------------------------------------------------------------

Thanks.
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Marty R.



Joined: 12 Feb 2006
Posts: 5176
Location: Rochester, NY, USA

PostPosted: Sat May 27, 2006 7:26 pm    Post subject: Reply with quote

There is a naked 42 pair in column 9. Since those two cells with the pair must contain a 4 and 2 in some combination, no other cells in column 9 can contain either a 4 or a 2. Since r6c9 can't have a 4, the only remaining option is the 8.
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gf1701



Joined: 27 May 2006
Posts: 4

PostPosted: Sat May 27, 2006 7:48 pm    Post subject: Reply with quote

Thanks for pointing that out. I don't know why I missed it.
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Marty R.



Joined: 12 Feb 2006
Posts: 5176
Location: Rochester, NY, USA

PostPosted: Sat May 27, 2006 9:32 pm    Post subject: Reply with quote

You're welcome. I can stare at a puzzle for hours and then finally see something that should have jumped right off the page at me. Happens much more than I'd like.
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