Pipes A and B can fill a tank in 12 minutes and 15 minutes, respectively. The tank when full can be emptied by pipe C in x minutes. When all the three pipes are opened simultaneously, the tank is full in 10 minutes. The value of x is:

A tank can be filled by A in 60 minutes and emptied by B in 75 minutes. The tank is half filled and Ravi wants to fill the tank but opens B mistakenly and realizes after half hour and closed tap B. He then opens A and tank is completely filled in _______ minutes.

One-fourth of a tank holds135 litres of water. What part of the tank is full if contains 180 litres of water?