Mental Arithmetic is not dead

So I went to buy my lunch today and the change came to some odd amount of cents.
"Bother", I thought, "A credit note".

Let me explain.
For the past few years, Zimbabwe has been a "multicurrency economy". What this means is that you can officially use US$, sterling, South Africa rand, Botswana Pula, Euro or even Zim$.  Officially.  In practice, in Harare, you use US$.
But we have notes only, not coins.  What coins are in circulation are rand or occasionally pula.
So if I hand over US$5 for a $4.32 burger and chips, the normal 68c change is not going to happen.  The normal is that the store will issue a credit note for 68c and the next time I am in that particular store (and remember), I cash it in.

Not this time.

I was handed, without so much as a pause, R2.05 and one pula.
Mental arithmetic is not dead!  Who in a normal world can calculate that USc 68 = R2.05 + P1 and calculate it in less time than that?

I might spend my days working in the Payment Industry and helping people do clever things with electronic money.  This helps the coin dilemma.
But it comes no where near close to the cleverness of the Mental Arithmetic at the shop.