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