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Struggling with the nine times table? I have a failsafe method (www.theguardian.com)

submitted 3 days ago by Aatube@lemmy.dbzer0.com to c/nottheonion@lemmy.world

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Apparently, nines are the hardest to grasp for primary school children. If only they’d learned how to cheat like me, says Adrian Chiles

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[–] Aatube@lemmy.dbzer0.com 2 points 1 day ago

that actually goes for every digit. when you add eight to a number n=10a+b=\overline{ab} (so a is in the tens digit and b is the ones digit), you can think of this as:

add all of the eight to b
if b is greater than 10 as a result, subtract 10 from b and add 1 to a

what is 8-10? -2. so if you're shifting the tens digit (a) up by 1, the ones digit is shifting down by 2; if you're looking at the sum of digits, +1-2=-1.

you can do similar things for every other digit added to a number. 8 is -2, 9 is -1, 7 is -3...