Comparison of Book Sales Between Weekdays and Weekends

A small bookstore tracks the number of books sold each day over a week. The sales data for the last week is as follows:

Monday: 20 books
Tuesday: 25 books
Wednesday: 30 books
Thursday: 15 books
Friday: 28 books
Saturday: 40 books
Sunday: 18 books

If we compare the number of books sold on weekdays (Monday to Friday) to the number of books sold on the weekend (Saturday and Sunday), which group sold more books?