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 ?

18 men can complete a certain work in 15 days working 8 hours a day. How many men can complete the same work in 12 days working 9 hours a day?

15 persons working 6 hours per day earn Rs. 900. How much money will 20 persons earn, working 10 hours per day?

8 children and 12 men complete a certain piece of work in 9 days. If each child takes twice the time taken by a man to finish the work, in how many days will 12 men finish the same work ?