Plus Fifteen by Robert Currie FREE

  

Plus Fifteen comes from Robert Currie.  The object is to link up numbers that add up to 15.

  

Each level has exactly 1 solution. Grid sizes go from 6x6 to 9x9.

I like the graphics a lot.  For some reason, you can't redo a level that's already been solved.  Hmmm.

The fact that there is only 1 solution for each level leads to some interesting analysis.  Look at the first image. The top row has 11 & 4 directly in the center. Under it is 4 & 11.  If the top 11 pairs with the 4 below it or to its right, then the other 11 must pair with the 4 that's left or above it.  But because there's only 1 solution, we know that those pairings would lead to two possible solutions.  Therefore, that top 11 must pair with the 4 on its left.

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