????????
      +---------
UNYGQ | PQGNQTXM
       -UNYGQ
        =====
        UGWXRT
       -UWGLPQ
        ======
         UHFHYX
        -UHPPFM
         ======
           MXPFM
          -LMHUM
           =====
           ?????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: hard
missing: J