Earlier quoted context omitted.
This is a common misconception. ChatGPT is not supposed to be good at this — it's a language model, not a maths model or data science model or whatnot. This is exactly why they have plugins, such as the one for Wolfram Alpha.
Of course people who actually know how ChatGPT works don't expect it to be able to magically solve mathematical problems. However, these examples do show that ChatGPT isn't (contrary to some of the hype) deriving a deep conceptual understanding of its input data.
No mathematician would make this exact mistake because it looks ridiculous, but for example I made similar mistakes multiple times, claiming low numbers prime or not prime wrongly during a reasoning. Especially when working at night.