Basic mathematics

must still be taught in schools but I wondered about this yesterday. On my way to the first knitting meeting of the year I stopped to pass on several things to a regular on my bike route. 

Her grandson was there. He's about fourteen and he was doing various things for her. He's one of those "good kids" who appears at her place on Saturday mornings "in case you need something done Gran".  

"You'll need twenty-seven of those at five metres each," his grandmother was saying as I went in.

The kid pulls out his phone and I know he is going to do the calculation on it so I say, "What she means is that you will need one hundred and thirty five metres."

He looked at me - and did the calculation on his phone.

"How did you know that?" he asked in disbelief.

"There's nothing smart about it - just a trick. Multiply by ten and divide by two. It works all the time."

He thought about it and then grinned at me, "Yeah, they should teach us stuff like that."

I know why they don't but then perhaps they should when students have reached an understanding of what "five" is? 

One of my statistics lecturers at university had the ability to do what people thought of as a sort of "party trick". She could multiply four figures by four figures "in her head". There was no trick involved. D.... simply used a system she had been taught by a man called Jakow Trachtenberg. He had been a prisoner of war and used his time in incarceration to develop a new way of working with numbers. D...had met him many years before she tried to teach me anything. She explained the method to the class one day but didn't actually teach us how it was done. What she did do was "remind" us of all sorts of ways doing basic mathematics.  

I don't do many mathematical calculations outside knitting and other craft and, of course, the shopping. There are still a few basic statistical tests I can apply but I have forgotten most of it. I have never needed it but, on the rare occasions that something like the need to multiply a larger number by five comes up, I can still do some things "in my head".  Perhaps we should teach those things?