???????
      +--------
NTQRL | VVUPWYP
       -XWUTP
        =====
        ZTJXPY
       - HQGHL
        ======
         TTZYXP
        -ZHPHQW
         ======
         ??????

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