????????
      +---------
YTJRR | UXNTTXXX
       -YTJRR
        =====
        TQWHPX
       -TJRGMM
        ======
         VZPJLX
        -VNHRGP
         ======
          NTTLHX
         -JZTRMY
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: medium