There are two types of equations:
1) Equivalet Equations
Given any equation, an equivalent equation will result if the same operation is done to both sides of the equation.
x+21=36
x+21-21=36-21
x+0=15
x=15
2) Inverse Operations
Two operations are called inverse operations if one reverses the effect of the other.
x+47.6=132.32
x+47.6-47.6=132.32-47.6
x+0=84.72
x=84.72