Two pipes can fill a tank in 30 minutes and 20 minutes

Two pipes can fill a tank in 30 minutes and 20 minutes. An outlet pipe can empty the tank in 15 minutes. If all the pipes are opened when the tank is empty, then how many minutes will it take to fill the tank?

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 #74387