This is part of a series on puzzle solving techniques. If you are stuck on a particular Krazydad puzzle, drop me a note, and I’ll use this space to help you out.

This is a tough puzzle, book 7, puzzle 7. It was submitted by Stephen, who got stuck after solving 4 numbers, like so:

If you’d like to try the puzzle yourself, before reading the rest of this, you’ll find the original here.

When I get stuck, I often look at one number at a time, looking for patterns. Let’s take a look at all the cells that appear to have 1 as a possibility. Am I missing any 1s?

Okay, looks like I got them all. That’s a lot of 1s! Now take a closer look at rows 1 and 4.

You’ll notice that in those rows, the 1s form the corners of a square. No other cells, but those 4 corner squares can take on the value of 1. This formation is called an X-Wing. It means that if C1 is 1, than F4 must be 1. If F1 is 1, then C4 must be 1. Thus, in both the rows and columns that are occupied by these squares, these are the only 4 cells that can take on the value of 1.

The X-pattern formed by the relationships between these 4 squares is where the ‘X’ in X-Wing comes from (and the columns and rows that shoot out are the “wings”). It means that in the columns, we can also eliminate 1 as a possibility from C2, F2, C5, F5 and C8. Having eliminated C8 as a possibility, we can now fill in the 1 for A8.

Can you see where to put a 4 now?

This entry was posted on Monday, August 29th, 2011 at 11:53 am and is filed under Puzzles. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.