Xiao Zhang practices calligraphy. He wrote 150 more words in the first four days than in the last three days, and wrote 45 more words in the last three days. He asked Xiao Zhang how many words he had written four days before.
Solution: Suppose you write X words every day for the first four days, then
4x-3(45+x)= 150，
∴x= 150+ 135=285
Xiao Zhang wrote 285 words every day for the first four days.