A father said to his son, "I was as old as you…

A father said to his son, "I was as old as you are at present at the time of your birth." If the father's age is 50 years now, the son's age 10 years back was?

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 #5315