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 Description  
 Forming an Equation (Writing an Equation):  Writing information presented in words as a mathematical sentence with an equality sign, operation symbols, numbers, and/or variables.
For example,

Suppose each audio CD costs $12.00 at a certain store. Let's write an equation to determine the total cost (without tax) for the number of CDs you want to purchase.

c = 12n

c is a variable representing the total cost and n is the variable representing the number of CDs purchased.

To find the solution of word problems, we usually form an equation to solve it. For example consider the follwing problem:

"The length of a rectangle is three times the width. The perimeter is 16 feet. What is the width of the rectangle?"

In this problem, we do not know what the length and width of the rectangle are. In other words they are the unknowns. But we do know the relations between those two. So, let's denote the width by "W" and the length by "L." We can write that
L = 3W
because it is stated that the length of the rectangle (L) is three times the width (W). Furthermore, we can also write the equation
2L + 2W = 16 or L + W = 8
because we are informed that the perimeter of the rectangle is 16 feet. So by writing/forming two equations,
L = 3W
2L + 2W = 16
we have a system of equations whose solutions for W and L gives us the length and width of the rectangle respectively.