Originally Posted by Ametrine
... add three to the seven making it a ten... What is this? ... for subtracting 8's and 6's... adds either a 2 or 4 respectively and completes the equation just as he does for the 7's.
Is this the complements method of subtraction (subtraction by addition)?
The examples above look like tens complements.
(Math is fun link- http://www.mathsisfun.com/numbers/subtraction-by-addition.html)