????????
      +---------
YJGYF | GYHPFMTU
       -YJGYF
        =====
        LUTWMM
       - YJGYF
        ======
         TFMHLT
        -TWURLT
         ======
           GRMMU
          -YJGYF
           =====
           ?????

base: 13
letters: FGHJLMPQRTUWY
missing: Q
difficulty: easy