????????
      ----------
YXYRE ) EQRNXWPR
       -EXRNP
        =====
         FPPYW
        -FYXXY
         =====
          QPYRP
         -QYYRR
          =====
           QXXWR
          -FYXXY
           =====
           ?????

base: 10
symbols: CEFNPQRWXY
missing: C
difficulty: medium