I am stuck at solving the following problem.
A certain hotel has 1,400 single rooms and 420 double rooms. Each room is cleaned by one person. If one person can clean a single room every 15 minutes and a double room every 20 minutes, how many cleaning persons are needed to clean all of the rooms if each person works for exactly 7 hours?
(A) 65
(B) 70
(C) 80
(D) 90
(E) 265