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