If you are a Maths teacher and you go shopping with a friend, probably you will be asked to do the calculations like finding the price after discount. Why ask me, you said. 'You teach Maths, right?' Some people assume that Math teachers are Maths wizards.
All of us study Maths in school and we take it at O level. We know the basics, so you don't need to be a Maths teacher to calculate things like 20% of $65. Most of us can still remember how to do it, right?
I think you are aware that hawkers at wet markets can do their calculations very fast. For example if the fish is $9.50 per kg. The fishmonger puts a fish on a simple weighing scale which only reads the weight and he tells you almost immediately "400g, $3.80, I sell you $3.50". I guess it is not very accurate, maybe they read to the nearest 50g.
I think it is because of frequent practice or perhaps they have memorized tables of weights and prices like we used to memorize our multiplication timetables when we were in school.