Problem Solving

At a constant hourly rate, Mark had estimated that he could read a book of 450 pages in a designated time, and he started reading the book. However, Mark took 3 hours longer than he had estimated, and, consequently, he finished 5 pages per hour less than his constant hourly rate. What was the time, in hours, that Mark had estimated for reading the complete book?

(a) 15
(b) 18
(c) 20
(d) 23
(e) None

OA A

Source: e-GMAT