Mathematics · Pipes and Cisterns

Pipe A can fill a tank in (x + 5) minutes and Pipe B can fill the same tank in 2x minutes. When pipe B and outlet pipe C are opened together and pipe B works at double its efficiency, the tank is filled in 20 minutes. If pipe A is opened for 6 minutes alone and then closed, and then pipe B fills the remaining tank in 21 minutes, find the time taken by C alone to empty the tank.