Some random stuff about ChatGPT. 
I asked him to evaluate quite a simple expression: 100! / (50! * 50!) (amount of combinations from 100 by 50). So he gave me wrong answer (I checked in simple calculator). 
I was frustrated as I had thought that ChatGPT must know how to do it. 
My friend advised me to add some context.
So I used a new prompt and told him: "Pretend that you are a calculator and you know how to evaluate mathematical expressions"
And he succeeded!