Three taps A, B and C can fill a tank in 12, 15 and 20 hours respectively. If A is open all the time and B and C are open for one hour each alternately, the tank will be full in:
P, Q and R together can do a piece of work in 56 days. P and Q together can do one-fourth as much work as R alone. In how many days can P and Q together complete the entire work?