GPT
       +---------
PRRMXG | XRXRGGRG
        -PRRMXG
         ======
          RTWWMR
         -QRWMWP
          ======
          GQMQPQG
         -GXRTGTT
          =======
           GGGGMQ

base: 8
letters: GMPQRTWX
difficulty: easy
restrictions: no multiplication