This post features hidden and naked subsets in two Sudoku X-treme review puzzles. All occur in line markings. The first continues from the previous post.

The X-treme 210 line marking features two naked triples. The first one breaks up a discouraging train of clueless lines.

The second follows soon thereafter in the closing the first column. Below is a trace of the collapse which follows.

The X-treme review puzzles also demonstrate their fair share of hidden subsets.

In X-treme165, the very first line marking, shown below, is a very obvious hidden single. There is only one location permitted for a 6. Of course the always complementary naked triple is no less obvious.

Box/line restrictions are also about available cell locations along the line. After four more lines, the marking of r9 confines 5’s to the SW box, denying them to SWr7 and confirming SE5.

This box/line could easily have been noticed earlier, but it is the line marking procedure that assures that it is seldom missed.

Two lines later in line marking X-treme 165, we encounter another subset situation. In r8, do we have a naked quad or a hidden single? The answer is, it’s both. Whichever you find easier to recognize.

The new clues 7r8c5 and 7r8c4 trigger the box/line eliminating 1r9c8, and shortly thereafter we have a surge of eliminations as 5r4c7’s naked triple generates two naked pairs. Here is the “before”. Want to mark it from there? Next post will show the “after” that sends X-treme 165 into collapse.

If you have it, dust off your copy of Peter Gordon’s Mensa Guide to Solving Sudoku. Very recently, the Mensa association was dropped, with the re-issue under a Sterling Publishing imprint PuzzleWright. We’ll be doing a detailed critique of Gordon’s instruction and examples, in two parts. The basic solving part continues this basic clinic. The advanced solving part – and a review of the puzzle collection by Frank Longo – both follow later, after the basic clinic is completed.

I’m hurrying the basic part of the Gordon review into the basic clinic to get to a related strategy for timed solving contests of basic level puzzles. The next  Akron, Ohio Sudoku tournament is coming up.

Wish me luck Saturday. I’m running the Akron marathon.