Amos is reading a 400 page book. On Monday, he reads 40 pages. On each day after the first, the number of pages that he reads is 20 more than on the previous day. Amos finishes the book on
(A) Friday
(B) Saturday
(C) Sunday
(D) Monday
(E) Thursday
On Monday, Amos read 40 pages.

On Tuesday, Amos read 60 pages, for a total of $40+60=100$ pages so far.

On Wednesday, Amos read 80 pages, for a total of $100+80=180$ pages so far.

On Thursday, Amos read 100 pages, for a total of $180+100=280$ pages so far.

On Friday, Amos read 120 pages, for a total of $280+120=400$ pages so far.

Therefore, Amos finishes the 400 page book on Friday.

Answer: (A)