Math 7 ( Period 4)

Percent of Increase or Decrease 7.7

A percent of change tells how much a quantity has increased or decreased relative to the original amount. you can use a ratio to find the percent of change.

change/ original

AGAIN—use the LADYBUG METHOD

When the new amount is greater than the old amount then you have a percent of increase. When the new amount is less than the old amount, you have a percent of decrease.

The enrollment in a middle school was 400 students for 1999, and 420 for 2000. Find the percent of change form 1999 to 2000.

LADYBUG-  find the difference 420 -200 = 20

20/400 = 2/40 = 0.05 = 5%

The percent of change was 5%

You place a 5 in by 7 inc photograph in a photo enlarger. You want to enlarge the photograph so it measures 7 inches by 9.8 inches. What enlargement setting should you use?

Use either the width or the length of each to find the percent of increase.

I want to use  the width

Enlarged width- original width
original width

7 -5 = 2

That’s the change

2/5 = 4/10 = 40%

The percent of increase is 40%. This means that the enlarged size  is 100% + 40% = 140%

Original grade =80 and new grade is 90

What’s your percent of increase

10/80 = 12.5%

What happens if your original grade was 90 and your new grae was 80  ( oh dear…)

10/90 = 11 1/9% decrease

Why are the %’s difference when the change is the same ( it was 10 in both cases) The c Percent of Change is based on WHAT YOU STARTED WITH.

Okay how about an original grade of 20 ( yes I said 20%-- Yikes…)

New grade is 30

30-20 = 10

What was the original? 20

so 10/20 = 50% Now, I know that this student is still failing but he ( she) has increase the grade 50% from what it had been.

This student can tell his parents that he increased his grade by 50% 
What his parents should ask is “From what?”

This is how a percent of change can be misused,  by advertisers.

Have you ever heard of  “ your teeth will be whiter by 50%.”  50% of what?

Or “Your gas mileage will increase by 50% if you use … gas..” Again, ask… 50% increase of what?