<pre>
                    ABCD
                    CABD
                    ACDB
                    DACB
                    BCDA
                    ACBD
                    ADCB
                    CDAB
                    DABC
                    BCAD
                    CADB
                    CDBA
                    CBAD
                    ABDC
                    ADBC
                    BDCA
                    DCBA
                    BACD
                    BADC
                    BDAC
                    CBDA
                    DBCA
                    DCAB
</pre>

Listed above are &nbsp; all-but-one &nbsp; of the permutations of the symbols &nbsp; '''A''', &nbsp; '''B''', &nbsp; '''C''', &nbsp; and &nbsp; '''D''', &nbsp; ''except'' &nbsp; for one permutation that's &nbsp; ''not'' &nbsp; listed.  


;Task:
Find that missing permutation.


;Methods:
* Obvious method: 
         enumerate all permutations of   '''A''',  '''B''',  '''C''',  and  '''D''',  
         and then look for the missing permutation. 

* alternate method:
         Hint:  if all permutations were shown above,  how many 
         times would  '''A'''  appear in each position?     
         What is the  ''parity''  of this number?

* another alternate method:
         Hint:  if you add up the letter values of each column, 
         does a missing letter   '''A''',  '''B''',  '''C''',  and  '''D'''   from each
         column cause the total value for each column to be unique?


;Related task:
* &nbsp; [[Permutations]])
<br><br>

