A number when divided by a divisor leaves a remainder of 21

A number when divided by a divisor leaves a remainder of 21. When twice the original number is divided by the same divisor, the remainder is 11. What is the value of the divisor?

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 #41530