Percent of Change

The percent that a value increases or decreases

Vocabulary (Section 5.7 in book)

• A percent of change is:– a ratio that compares the change in a

quantity to the original amount.

• For a percent of change, the proportion used is:

amount original

change ofamount

100

change ofpercent

Ex 1) The Mitchell family bought a house several years ago for $120,000. This year, they sold it for $150,000. Find the percent of increase in the price of the house.

000,120$000,120$000,150$

100x

A percent of increase is a percent of change.

Subtract the house values to find the amount of change.

The price of the house increased by 25%.

amount original

change ofamount

100

change ofpercent

Use the starting price as the original amount.

%25

)000,30(100000,120

000,120$000,30$

100

x

x

x

Ex 2) Mercer Middle School had 240 students last year. This year, there are 192 students enrolled. Find the percent of change in the number of students. Is this a percent increase or a percent decrease?

240192240

100x

The percent of change is a percent of decrease.

Subtract the number of students to find the amount of change.

The number of students decreased by 20%.

amount original

change ofamount

100

change ofpercent

Use the earlier number of students as the original amount.

%20

100)48(24024048

100

x

x

x

Ex 3) The average speed of the winning race cars in the Indy 500 are shown in the table. Find the percent of change in speed from 1955 to 2010. Round your answer to the nearest whole percent.

128128162

100x

The percent of change is a percent of increase.

Subtract the speeds between 1955 and 2010 to find the amount of change.

The average speed increased by approximately 27%.

amount original

change ofamount

100

change ofpercent

Use the average speed from 1955 as the original amount.

%27

100)34(12812834

100

x

x

x

Year Speed (mph)

1922 94

1955 128

2010 162