The number of books borrowed on Friday is higher by 40 * 40/100 = <<40*40/100=16>>16 books.
There are 5 days from Monday to Friday inclusive, so Krystian borrows an average of 5 * 40 = <<5*40=200>>200 books during that time.
With Friday's increase in borrowings, during one week Krystian borrows 200 + 16 = <<200+16=216>>216 books.
#### 216