You're asking a language model to do math. What's impressive there is not that it fails but that it comes up with an answer at all, especially if it is in the ballpark.

Most humans would do exactly the same unless given either access to pen and paper or a calculator, and it would likely be trivial for GPT-3 input processing to detect it has been presented with a math question and to farm it out to a special calculation module. Once you start to augment its input like that progress would be very rapid but it would no longer be just a language model.