????????
      +---------
GPMYJ | MLJXVUPQ
       -GPMYJ
        =====
        PMXMZU
       -PJUNVR
        ======
         HZYYUP
        -HJLVHY
         ======
          XLMQHQ
         -XGGRWY
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: challenging
hints: L is even
missing: T