Pipe P and pipe Q are inlet pipes, while pipe R is an outlet pipe. Pipe P can fill four-fifths of a tank in 24 hours, and pipe Q can fill two-ninths of the same tank in 10 hours. Both P and Q are kept open for 3 hours and then closed. Pipe R alone is then opened, and it empties the water in the tank in 6 hours. If pipes P, Q, and R are opened together, in how many hours can they fill the empty tank? 9450