Inverse
Inverse means the opposite in effect. The reverse of.
The Inverse of Adding is Subtracting
Adding moves you one way, subtracting moves you the opposite way.
Example: 20 + 9 = 29 can be reversed by 29  9 = 20 (back to where we started)
And the other way around:
Example: 15  3 = 12 can be reversed by 12 + 3 = 15 (back to where we started)
The Inverse of Multiplying is Dividing
Multiplying can be "undone" by dividing.
Example: 5 × 9 = 45 can be reversed by 45/9 = 5
It works the other way around too, dividing can be undone my multiplying.
Example: 10 / 2 = 5 can be reversed by 5 × 2 = 10
But Not With 0
You can't divide by 0, so don't try!
Example: 5 × 0 = 0 cannot be reversed by 0/0 = ???
