Two pipes A and B can fill a cistern in 12 and…

Two pipes A and B can fill a cistern in 12 and 20 minutes respectively, and a third pipe C can empty it in 30 minutes. How long will it take to fill the cistern if all the three are opened at the same time?

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 #4444