The tens complement is the number needed to make a single digit add up to 10. This technique is a building block for use in other tricks.
Example
10’s complement of 7?
= 3
Step 1
Subtract the number that you are finding the 10’s complement of from 10.
10 - 7 = 3
Step 2
The result of step 1 is the 10’s complement.
3 is the 10’s complement of 7.