????????
      +---------
XYRAW | AAXWWFQY
       -CWXCA
        =====
        YXFQCF
       -YYTYAA
        ======
         RWWARQ
        -RYAQXC
         ======
          RCMARY
         -RYAQXC
          ======
          ??????

base: 10
letters: ACFMQRTWXY
difficulty: intermediate