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The inverse of adding 5 is subtracting 5; the inverse of multiplying by 4 is dividing by 4. The inverse undoes or reverses what has taken place.

The inverse of a number is 1 divided by the number.
The inverse of a function or a transformation is the function or transformation which 'undoes' it.
The inverse of an element of a set, or a number, with respect to a particular operation, is what you have to combine the element with in order to get that operation's identity element.
eg:
The inverse of 5 is 1/5
The inverse of addition is subtraction
The inverse of a clockwise rotation is an anticlockwise rotation.
The additive inverse of 5 is -5, because 5 + (-5) is 0, which is the additive identity.
The inverse of the inverse always gets you back to where you started.