A man can do a job in 15 days

A man can do a job in 15 days. His father takes 20 days and his son finishes it in 15 days. How long will they take to complete the job if they all work together?

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 #47851