Half a page is 20 / 2 =<<20/2=10>>10 lines.
In total, he has written 20 + 10 = <<20+10=30>>30 lines.
The 30 lines written are 30 * 10 = <<30*10=300>>300 words.
Lucas has 400 - 300 = <<400-300=100>>100 words left to write.
#### 100