Mental arithmetic has almost completely gone out of fashion. Why would you though, now everyone has a mobile phone? It's hard to imagine now that there once was a time we could not even count. But at some point we made ourselves familiar with all kinds of skills. That for example went like this: as prehistoric man we learned to make all kinds of tools and aids. Life became easier. Everyone made his own stone axe to chop branches off trees or to slice a fish into pieces. But we could also already make fishhooks from bones. Then came a time that we were started to exchange those articles. Some men were skilful in making a knife out of stone, others in the plaiting of baskets. Exchanging always involves numbers. How many baskets you can ask for a stone knife? And how to make that clear to the basket-maker?
So we started to get a little idea for numbers and began to make that clear by putting up the number of fingers, so for example 2 fingers for the number of knives against 3 fingers for the number of baskets. But that was not enough, of course, we had only ten fingers. Then someone came up with the idea of putting scratches on a branch or a bone. We know that because a bone with scratches of 22000 years old has been found in Ishango, a place in the then Belgian Congo. That was the beginning of characters representing numbers. And 5000 years ago man was already so advanced that, by pressing nail heads into wet clay tablets, he could express numbers in a 60 base numeral system. That cuneiform script was later reverted to the current ciphers. That was very handy because we could memorise the tables of multiplication off by heart but 321x 685 does not work out in mental arithmetic and a piece of paper is needed to write down after each other 5x1, 5x2 and 5x3 and count up the results from the memorised tables in our head and add together and repeat this for 8 and 6. So, there is still mental arithmetic’s involved. Although? Who actually really wants to calculate 321x 685? But in case you like the fun of the challenge or do not have a mobile phone handy, you know again now how it works.