????????
      +---------
YNYCU | UGCRNTFC
       -UNCRF
        =====
         QFFYT
        -QYNNY
         =====
          GFYCF
         -GYYCC
          =====
           GNNTC
          -QYNNY
           =====
           ?????

base: 10
letters: CFGNQRTUXY
missing: X
difficulty: medium