Survo puzzles: www.survo.fi/puzzles
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Copyright (c) Survo Systems 2006-2007

Survo puzzle 78/2007 (55)

   A  B  C  D
1  *  *  *  *  53
2  *  1  2  *  22
3  *  3  4  *  30
4  *  *  *  *  31
  40 17 35 44

Survo puzzle 79/2007 (175)

   A  B  C  D
1  *  *  *  *  25
2  *  1  *  *  35
3  * 15  2  *  33
4  *  *  *  *  43
  38 24 44 30

Survo puzzle 80/2007 (17500)

   A  B  C  D  E
1  *  *  *  *  *  20
2  *  *  *  *  *  44
3  *  *  *  *  *  65
4  *  *  *  *  *  81
  24 33 38 57 58

The degree of difficulty (17500) of the problem 80/2007
indicates that it is a hard task to solve especially
when trying to show that it has only one solution.
If showing uniqueness of the solution is not required,
the swapping method may be an option worth of trying.