Two pipes P and Q can fill a tank in 4 hours…

Two pipes P and Q can fill a tank in 4 hours and 5 hours respectively. If both pipes are opened simultaneously, how much time will be taken 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 #1340