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Algebra - Two Variables
Variable Substitution
You can solve a system of equations with a method called substitution. Here’s how to use it.
You have two equations, so “solve†one equation by writing A in terms of B. That is, use the
rules of algebra to rearrange the equation to get “A =†on one side. Then substitute this
expression into the other equation wherever variable A appears. This results in one equation
and one unknown.
Example:
y = 4x
x + y = 90
What are the values of x and y?
Solution:
The first equation y
= 4x tells you that “4x†is another name for y.
Substitute 4x for y:
x + y = 90
x + 4x = 90
5x = 90
Divide both sides by 5:
5x/5 = 90/5
x = 18
To find y, substitute 18 for x in either
of the original equations:
y = 4x
y = 4(18)
y = 72
Substitute the values of x and y into
the other equation:
