The nines complement is the number needed to make a single digit add up to 9. This technique is a building block for use in other tricks.
Example
9’s complement of 7?
= 2
Step 1
Subtract the number that you are finding the 9’s complement of from 9.
9 - 7 = 2
Step 2
The result of step 1 is the 9’s complement.
2 is the 9’s complement of 7.