I was helping my daughter with her 8th grade math last night, and she was
marveling at my ability to do so much in my head. I told her that it was
practice, and some ‘tricks’. (I didn’t mention that I thought it might
also be that she relies on a calculator too much.)
The one trick I showed her that totally baffled her was what we used to
call the ‘Rule of 9’s’: If you have a 2 digit number, AB, and are
subtracting off a number, BA, the result is just abs(A-B)*+/-9. It takes
longer to write it out than to do it in your head.
Example 19 -91 =?
Abs(9-1)*-9=-72
She was amazed by this, and wondered why her teachers had never shown her
this. Then, she asked me if there was a name for this and who thought of
it. This is where I am stumped, is there a ‘real’ name for this? I don’t
think “Rule of 9’s” is a true name. Also, do you know who might of first
thought of this? (I asked my colleagues in the math dept. and they had
never heard of this.)