The Algebra Teacher\'s Guide to Reteaching Essential Concepts and Skills

Teaching Notes 2.18: Finding the Percent of Increase

or Decrease

Although related, an increase and percent of increase are quite different, as are a decrease and

percent of decrease. Students often mistake an increase or decrease for a percent of increase or

decrease.

1. Explain that a percent of increase is a rate per hundred that equals the ratio of the increase
   to the original amount. Similarly, a percent of decrease is a rate per hundred that equals the
   ratio of the decrease to the original amount.


2. Review the information and example on the worksheet with your students. Explain the
   formulas. Be sure that your students understand that both the increase and decrease are
   compared to the original amount. Discuss the example and point out that the student’s aver-
   age increased 10 points from his original average. Although the example illustrates finding
   the percent of increase, point out that the percent of decrease is found in the same manner,
   except that the decrease (instead of increase) is placed over the original amount in the for-
   mula.


3. Depending on the abilities of your students, you may find it helpful to review the steps for
   solving proportions and rounding to the nearest percent.

EXTRA HELP:

The only difference in the formulas for finding the percent of increase or decrease is the numerator

of the second fraction of the proportion. For finding the percent of increase, the numerator is the

‘‘amount of increase.’’ For finding the percent of decrease, the numerator is the ‘‘amount of decrease.’’

ANSWER KEY:

(1)23% decrease (2)11% increase (3)10% decrease (4)8% decrease

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(Challenge)No. Mike received about an 8 percent increase while Juan received about a

------------------------------------------------------------------------------------------6 percent increase.

82 THE ALGEBRA TEACHER’S GUIDE