Chapter 6—Expressions and Equations

6-6 Working With Formulas

When you complete the work for this section, you should be able to:
  • Describe the purpose of a formula.
  • Demonstrate the use of formulas.



A formula is a mathematical statement, or equation, that shows the exact relationship between two or more variables.

A formula is very much like a set of instructions for converting variables into a solution.  Here is an example of a formula that expresses distance traveled in terms of the average speed and time of travel. The formula looks like this:

d = st


d = the distance traveled
s = the average speed
t = the amount of time

If you know the speed and time of travel, the formula provides the exact instructions for finding the distance:  multiply the speed by the time. It's simply a matter of evaluating the expression a formula in this case.


A car travels an average speed of 45 miles per hour for 4 hours. How far does it travel in that time?

The desired formula is d = st.
Substituting the known values:  d = 45 x 4
Doing the math: d = 180 miles.

The formula for converting Centigrade temperatures to Fahrenheit is:

F = 1.8C + 32


F = temperature in F
C = temperature in C


Express 50C to Fahrenheit.

The desired formula is F = 1.8C + 32
Substituting known values: F = 1.8(50) + 32
Doing the math: F = 90 + 32 = 122

So: 50C = 122F