Two pipes can fill a tank in 4 hr and 5 hr respectively. If they are turned on alternatively for 1 hr each, the tank will be filled in

A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9 hours.If both the taps are opened simultaneously,then after how much time will the cistern get filled?

A and B can complete a work in 15 days and 10 days respectively.They started doing the work together  but after 2days B had to leave and A alone completed the remaining work.The whole work is completed in: