An equation is a mathematical statement such that the expression on the left side of the equals sign (=) has the same value as the expression on the right side. An example of an equation is 60 - 40 = 20.

One of the terms in an equation may not be know and needs to be determined. Often this unknown term is represented by a letter such as x. (e.g. x - 40 = 20).

The solution of an equation is finding the value of the unknown x. To find the value of x we can use the additive equation property which says: The two sides of an equation remain equal if the same number is added to each side.

Example:
x - 50 = 70
x - 50 + 50 = 70 + 50
x - 0 = 120
x = 120
Check the answer by substituting the value of x (120) back into the equation.
120 - 50 = 70