A man can do a piece of work in 5days, but with…

A man can do a piece of work in 5days, but with the help of his son, he can finish it in 4days. In what time can the son do it alone?

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 #1317