P, Q and R are three typists who working simultaneously can type 216 pages in 4 hours. In one hour, R can type as many pages more than Q as Q can type more than P. During a period of five hours, R can type as many pages as P can during seven hours. How many pages does each of them type per hour ? `14 , 17 , 20` b. `15 , 17 , 22` c. `15 , 18 , 21` d. `16 , 18 , 22`