???????
      +--------
PEPWW | QGGQWGX
       -PEPWW
        =====
        YXQEFG
       -XEQWXF
        ======
         GQUQGX
        -GPYPQW
         ======
         ??????

base: 10
letters: CEFGPQUWXY
difficulty: easy
missing: C