The mean of 50 observations is 100

The mean of 50 observations is 100. But later he found that there is decrements of 13 from each observations. what is the the updated mean is ?

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 #13119