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[Clement]

XY-Wing 24 27 47 eliminates 4 in r3c1 solves the puzzle.

[Toole130]

I noticed the connection of 45 (R3C1) to 24 (R3C9) to 27 (R7C9) to 47 (R7C1). I did not know what to call it or how to pick out the number to be eliminated. I just chose 4 for R3C1, went around the loop, and found this would not work. So this cell must be 5. Is there a more sophisticated way to arrive at which cell is the "odd" cell? Is it the one containing a number that is not in the other 3 cells?

Bart

[Marty R.]

What you have is an XY-Wing, the one that Clement mentioned. The basic rule of this technique is you start with a cell we'll call XY. That cell must see both a cell we'll call XZ and YZ. Any cell(s) that see both XZ and YZ cannot be Z.

XY = 27
XZ = 24
YZ = 47

Since r3c1 sees both the 24 and 47 it cannot contain a 4.

Hope this helps, ask away if necessary.

[Wendy W]

Nice XY-planation, Marty!
I used Clement's XY-wing but first I needed another XY-wing, 278.