Two pipes A and B can fill a cistern independently in 12 min and 15 min, respectively. A pipe C at the bottom of the cistern empties the tank. When all the three pipes are opened, the cistern will be filled in 20 min. A pipe D fixed at the bottom of the cistern working along with A, B and C will empty the full cistern in 20 min. The time taken by D (in minutes) to empty the full cistern independently is