ABCD
                    CABD
                    ACDB
                    DACB
                    BCDA
                    ACBD
                    ADCB
                    CDAB
                    DABC
                    BCAD
                    CADB
                    CDBA
                    CBAD
                    ABDC
                    ADBC
                    BDCA
                    DCBA
                    BACD
                    BADC
                    BDAC
                    CBDA
                    DBCA
                    DCAB
Listed above are   all-but-one   of the permutations of the symbols   '''A''',   '''B''',   '''C''',   and   '''D''',   ''except''   for one permutation that's   ''not''   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: *   [[Permutations]])