On 11/01/2019 13:45, NY wrote:
"Tim Streater" wrote in message
.. .
Everyone should be able to do mental arithmetic, and even more
importantly, be able to quickly work out an approximate answer to a
calculation, which will tell you whether your final calculation is
*reasonable*.

I wish I'd been taught how to do *mental* arithmetic and how to process
the carry/borrow digits and to retain a mental running total. I never
was: I was taught how to do it on paper, with rules for
carrying/borrowing digits which I can do fine (albeit slowly and
laboriously).

My wife worked in a bakery as a summer job during school, so she quickly
became adept at adding up prices of five doughnuts at 13p each, two
loaves of bread at 27p each, 7 seven flapjacks at 17p each, *without
having to write it down and add up on paper*. I marvel at that skill.

You need to develop shortcuts.

5 x 13 = 50 + 15 = 65
2 x 27 = 40 + 14 or 60 - 6 = 54
65 + 54 = 110 + 9 = 119
7 x 17 = 7 x 20 - 21 or 70 + 49 = 119
119 + 119 = 120 + 120 - 2 = 238

The more you do though, the less you need the shortcuts as you begin to
recognise that 2 x 27 is 54 and similar without any intermediate stage.

SteveW