John paid a sum of money for purchasing 20 pens, which he…

John paid a sum of money for purchasing 20 pens, which he recovered in full when he sold 10 of them. What was his percentage of profit or loss per pen?

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 #54350