A, B and C can do one-third of a work in 15 days, 30 days and 10 days, respectively. A started the work. C joined him after 1 day and B joined them after 3 days of the beginning. For how many days did C work?
Tap P can fill a cistern in 6 hours and tap Q can empty the full cistern in 10 hours. If both taps P and Q are kept open simultaneously, then in how many hours will the empty cistern be completely full ?