Hidden Pair: Imagine that in a row, column, or group there are only TWO boxes that could become 3 or 7. If those two boxes had NO OTHER CHOICES, then they would both be "doubles", or "naked pairs" and they would easily be spotted - a Level 4 skill.
But if one or both of them also allowed OTHER numbers, then they are "Hidden pairs". Finding them is often rewarding.
Double Line Eliminations: Imagine that in groups 1 and 2 you find boxes that allow 7s only in the top two rows.
This means that group 1 or group 2 must have 7 in row 1, and the other group will have its 7 in row 2.
Groups 1 and 2 are claiming those two rows for 7.
This means that ... if present...7's can be crossed out in the first two rows of Group 3.
An equally valid way to view this is that Group 3 contains the ONLY 7's that exist in Row 3, and so again, within Group 3 you can cross them out of rows 1 and 2.
To jump straight to practice of hidden pairs, jump down to the next showing of this puzzle.
PB Post 2013 05 19 Lvl 5 Sunday (Repeated lower down)
For no good reason, I'm presenting hints of history of my order of solving this puzzle.
First, I found a naked 9.
Then I found Hidden 2, Hidden 6, two hidden 7s, hidden 9,
two hidden 4s, ALL of the 7s, hidden 5.
Then, in row 3, I used "single line eliminations" to cross out some 5s in group 3.
That left me with 38 unsolved boxes, as shown below. A good place to practice finding hidden pairs.
PB Post 2013 05 19 Sunday Lvl 5 w38 Same as above, except the pink boxes are filled in.
In row 3, the two underlined 5's can be crossed out by "single line elimination".
In any order...you can now locate:
in Row 3 and in Column G: 2/4 Hidden pairs. which allow some eliminations.
in Group 4, find the 5/6 hidden pair, and eliminate more things.
FINALLY, in Row 5, there is now a hidden 3.
After all that, the puzzle finishes with all naked singles.
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