Some offices expect you to be more proficient in math than others, but even those that have higher expectations won't penalize you for somewhat slow math. There was a time when a manager during McKinsey interview asked me why it took me so long to calculate, I just said I didn't want to make a negligent mistake by missing a zero digit and arriving at 10x wrong result. I passed that interview.

You are not expected to perform a double-digit multiplication in you head.

It's expected that you'll be efficient by rounding numbers, finding shortcuts and not spending too much time trying to arrive at a precise result. For 78*83 they'll expect that you'd suggest to use 80*80=6,400 instead but can do a more precise calculation if need be. There is a 1% rounding error here, which is totally negligible and will allow you to arrive at the same "so what" conclusion as the 78*83=6,474.

(edited)