Let x be the number of books borrowed by lunchtime.
So by lunchtime, the remaining number of books will be 100-x
After adding 40 more books, the total number of books increased to 100-x+40.
By evening, after 30 more books have been borrowed, the total number of books remaining is 100-x+40-30 = 60
Thus 110 - x = 60.
Thus x = <<50=50>>50 books
#### 50