Two pipes A and B can separately fill a cistern in 20…

Two pipes A and B can separately fill a cistern in 20 min and 30 min respectively. There is a third pipe in the bottom of the cistern to empty it. If all the three pipes are simultaneously opened, then the cistern is full in 60 min. In how much time, the third pipe alone can empty the cistern?

This problem repeats another one in the library (at most with different numbers), so it's left out of browsing.

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #70693