Thanks for the advice chaps.
The grandchildren were still at primary school... work it out...
I guess it won't be long before a generation starts being born with iPads instead of brains inside their skull, pre-programmed.
I used a circular slide rule at the supermarket check out the other day - one stuck inside my wallet that was once a gimmick key ring, about 2" diameter - to prove that the girl had rung up the 2-for-1 price incorrectly. She asked in utter amazement what it was, and how had I come up with the right price so quickly, i.e. marginally quicker than she had cancelled the till entry and tried again. I started to explain, and show her - then gave up, waste of time.
I used to go around the S'market with an pocket electronic calculator, but one day it packed up in the middle of a long tally ! flat battery. Threw it away. A S'market chain in Singapore used to provide calculators built into the trolley handles - gave up, they kept getting pinched !