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Medium Puzzle #39 - One Possible Solution Approach…
NB:  There is only one final solution to this Sudoku puzzle…
A B C D E F G H I
1                  
2 AA BB CC Reference to 3x3 Blocks AA, BB & CC
3                  
4                  
5 DD EE FF Reference to 3x3 Blocks DD, EE & FF
6                  
7                  
8 GG HH II Reference to 3x3 Blocks GG, HH & II
9                  
A B C D E F G H I
1 2           5 8   Medium Puzzle #39 - November 8, 2005
2 4 9     8   2   1 Original puzzle as published © joe-ks.com
3       7 1     4  
4 9           4   8
5 3 7   4 6        
6       1 5   3   7
7 1             3  
8 6 8   9 4   7   2
9         2   9 5  
A B C D E F G H I
1 2           5 8   Step 1: Column G needs #1: #1 MUST go in Cell G5
2 4 9     8   2   1      (#1 in Row 3 @ E1 excludes use of Cell G3, &
3       7 1     4          #1 in Row 7 @ A7 excludes use of Cell G7)
4 9           4   8
5 3 7   4 6   1    
6       1 5   3   7
7 1             3  
8 6 8   9 4   7   2
9         2   9 5  
A B C D E F G H I
1 2           5 8   Step 2: #1 is now in Columns G & I: #1 MUST go in Cell H8...
2 4 9     8   2   1
3       7 1     4   Step 3: #1 is now in Rows 7 & 8: #1 MUST go in Cell F9
4 9           4   8      (#1 in Column D @ D6 excludes use of Cell D9)
5 3 7   4 6   1    
6       1 5   3   7
7 1             3  
8 6 8   9 4   7 1 2
9         2 1 9 5  
A B C D E F G H I
1 2           5 8   Step 4: Block II needs # 8: #8 MUST go in Cell G7
2 4 9     8   2   1      (#8 in Column I @ I4 excludes use of Cells I7 & I9)
3       7 1     4  
4 9           4   8 Step 5: #8 is now in Rows 7 & 8: #8 MUST go in Cell D9...
5 3 7   4 6   1    
6       1 5   3   7
7 1           8 3  
8 6 8   9 4   7 1 2
9       8 2 1 9 5  
A B C D E F G H I
1 2         4 5 8   Step 6: #4 is in Rows 2 & 3: #4 MUST go in Cell F1
2 4 9     8   2   1      (#4 in Column D @ D5 excludes use of Cell D1, &
3       7 1     4          #4 in Column E @ E8 excludes use of Cell E1)
4 9           4   8
5 3 7   4 6   1     Step 7: #6 is in Row 8 (@ A8 in Block GG): #6 must go in
6       1 5   3   7      Blocks HH in either D7 or F7… BUT that means that #6
7 1           8 3        in Block II MUST go in Block II, Row 9 - & it MUST be in
8 6 8   9 4   7 1 2      the only cell remaining - in Cell I9)
9       8 2 1 9 5 6
A B C D E F G H I
1 2   7     4 5 8   Step 8: Complete Block II: #4 MUST go in Cell I7...
2 4 9     8   2 7 1
3       7 1     4   Step 9: #7 is in Columns G & I: #7 MUST go in Cell H2
4 9           4   8
5 3 7   4 6   1     Step 10: #7 is now in Rows 2 & 3: #7 MUST go in Cell C1
6       1 5   3   7     (#7 in Column B @ B5 excludes use of Cell B1)
7 1           8 3 4
8 6 8   9 4   7 1 2
9       8 2 1 9 5 6
A B C D E F G H I
1 2   7     4 5 8   Step 11: Complete Column G: #6 MUST go in Cell G3...
2 4 9     8   2 7 1
3       7 1   6 4   Step 12: #7 is in Columns B & C: #7 MUST go in Cell A9...
4 9           4   8
5 3 7   4 6   1    
6       1 5   3   7
7 1           8 3 4
8 6 8   9 4   7 1 2
9 7     8 2 1 9 5 6
A B C D E F G H I
1 2   7     4 5 8   Step 13: Block HH needs #3: #3 MUST go in Cell F8
2 4 9     8   2 7 1      (#3 in Row 7 @ H7 excludes use of Cells D7, E7 & F7)
3       7 1   6 4  
4 9           4   8 Step 14: #5 is in Columns G & H: #5 MUSt go in Cell I5
5 3 7   4 6   1   5
6       1 5   3   7
7 1           8 3 4
8 6 8   9 4 3 7 1 2
9 7     8 2 1 9 5 6
A B C D E F G H I
1 2   7     4 5 8   Step 15: #9 is in Rows 8 & 9: #9 MUST go in Cell C7
2 4 9     8   2 7 1      (#9 in Column B @ B2 excludes use of Cell B9)
3       7 1   6 4  
4 9           4   8
5 3 7   4 6   1   5
6       1 5   3   7
7 1   9       8 3 4
8 6 8   9 4 3 7 1 2
9 7     8 2 1 9 5 6
A B C D E F G H I
1 2 1 7     4 5 8   Step 16: #1 is in Rows 2 & 3: #1 MUST go in Cell B1...
2 4 9     8   2 7 1
3       7 1   6 4   Step 17: #1 is now in Columns A & B: #1 MUST go in Cell C4
4 9   1       4   8      (#1 in Row 5 @ G5 excludes use of Cell C5, &
5 3 7   4 6   1   5        #1 in Row 6 @ D6 excludes use of Cell C6)
6       1 5   3   7
7 1   9       8 3 4
8 6 8   9 4 3 7 1 2
9 7     8 2 1 9 5 6
A B C D E F G H I
1 2 1 7 6   4 5 8   Step 18: Block AA needs #6: #6 MUST go in Cell C2
2 4 9 6   8   2 7 1      (#6 in row 3 @ G3 exludes use of Cells A3, B3 & C3)
3       7 1   6 4  
4 9   1       4   8 Step 19: #6 is now in Rows 2 & 3: #6 MUST go in Cell D1
5 3 7   4 6   1   5      (#6 in Column E @ E5 excludes use of Cell E1)
6       1 5   3   7
7 1   9     6 8 3 4 Step 20: #6 in now in Columns D & E: #6 MUST go in Cell F7
8 6 8   9 4 3 7 1 2
9 7     8 2 1 9 5 6