I just asked ChatGPT to multiply two 4-digit numbers, and two 7-digit numbers without external help. It got both right. I'm sure it wouldn't have a 100% success rate, but saying it can't do arithmetic is just false.
I tried prompt "6379 times 3875" and it was off by exactly 1000 on first try, and correct on second. 0% success rate, sample size of 1.
Are you sure it honoured your stipulation of "without external help"? For all we know, it hacked its way into Wolfram Alpha and got the result from there.
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I would be nice to see what the (unencrypted) reasoning trace is like. Multiplication with scratch paper is not particularly difficult.