2008 Gauss 8 Problem 14

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

\textbf{(A)} Friday \textbf{(B)} Saturday \textbf{(C)} Sunday \textbf{(D)} Monday \textbf{(E)} Thursday

If there are no answer choices shown, enter a numerical answer.

Full credit to this problem is given to the CEMC, you may view all Gauss contests here.