I can do it with a scratch pad. And I can also tell you when the calculation exceeds what I can do in my head and when I need a scratch pad. I can also check a long multiplication answer in my head (casting 9s, last digit etc.) and tell if there’s a mistake.

The LLMs also have access to a scratch pad. And importantly don’t know when they need to use it (as in, they will sometimes get long multiplication right if you ask them to show their work but if you don’t ask them to they will almost certainly get it wrong).