16. A tap can fill a cistern in 8 hours and another can empty it in 16 hours. If both the taps are opened simultaneously, the time ( in hour) to fill the tank is:

17. A tap can fill a tank in 16 minutes and another can empty it in 8 minutes. If the tank is already half full, and both the taps are opened together, the tank will be:

18. A pipe can fill a cistern in 12 minutes and another pipe can full it in 15 minutes, but a third pipe can empty it in 6 minutes. The first two pipes are kept open for 5 minutes in the beginning and then the third pipe is also opened. In what time is the cistern emptied?