Long multiplication is a trivial form of reasoning that is taught at elementary level. Furthermore, the LLM isn't doing things "in its head" - the headline feature of GPT LLMs is attention across all previous tokens, all of its "thoughts" are on paper. That was Opus with extended reasoning, it had all the opportunity to get it right, but didn't. There are people who can quickly multiply such numbers in their head (I am not one of them).
Long multiplication is a trivial form of reasoning that is taught at elementary level. Furthermore, the LLM isn't doing things "in its head" - the headline feature of GPT LLMs is attention across all previous tokens, all of its "thoughts" are on paper. That was Opus with extended reasoning, it had all the opportunity to get it right, but didn't. There are people who can quickly multiply such numbers in their head (I am not one of them).
LLMs don't reason.