????????
      +---------
MYNHN | NRXNQGXP
       -ZQVHJ
        =====
        RZQGQG
       -RZMRGP
        ======
          TXZTX
         -TVYHP
          =====
           PJWVP
          -QGWHL
           =====
           ?????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: challenging
missing: U