Five people borrowed 5 people * 2 books/person = <<5*2=10>>10 books on the first day.
After the second day, the total number of borrowed books increased to 10 books + 20 books = <<10+20=30>>30 books.
The total number of books on the shelf after the second day is 100 books - 30 books = <<100-30=70>>70 books
#### 70