I was rubbish at math at school and diverted into other subjects as soon as possible. Back in the day though we were taught arithmetic.
So I can add; subtract; multiply and divide in my head. This seems to amaze people under about 30.
We have a lot of small size, small value coins which accumulate in my jeans pocket.
I will often go into a shop, make a few small purchases that come to say, £3.47 and then count that out in 1p; 2p; 5p; 10p; and 20p coins in the palm of my hand.
This brings a look of amazement/fear to the eyes of the 17 year old behind the counter and they'll put it straight into the register without checking it.
It's lucky I'm honest but I am one of a dying breed.