Studying for her test, Mitchell had read ten chapters of a book before 4 o'clock. When it clocked 4, Mitchell had read 20 pages of the 11th chapter of the book she was studying from. After 4 o'clock, she didn't read the remaining pages of chapter eleven but proceeded and read 2 more chapters of the book. If each chapter in the book had 40 pages, calculate the total number of pages that Mitchell had read altogether?
Since each chapter of the book has 40 pages, Mitchell had read 10*40 = <<10*40=400>>400 pages from the first ten chapters.
After reading 20 pages of the eleventh chapter, the total number of pages that Mitchell had read is 400+20 = <<400+20=420>>420
The next two chapters that she read had 2*40 = <<2*40=80>>80 pages.
In total, Mitchell read 420+80 = <<420+80=500>>500 pages of the book that day.
#### 500