Elementary Algebra 2e

# 3.2Solve Percent Applications

Elementary Algebra 2e3.2 Solve Percent Applications

## Learning Objectives

By the end of this section, you will be able to:

• Translate and solve basic percent equations
• Solve percent applications
• Find percent increase and percent decrease
• Solve simple interest applications
• Solve applications with discount or mark-up

## Be Prepared 3.3

Before you get started, take this readiness quiz.

Convert 4.5% to a decimal.
If you missed this problem, review Example 1.106.

## Be Prepared 3.4

Convert 0.6 to a percent.
If you missed this problem, review Example 1.107.

## Be Prepared 3.5

Round 0.875 to the nearest hundredth.
If you missed this problem, review Example 1.94.

## Be Prepared 3.6

Multiply (4.5)(2.38).
If you missed this problem, review Example 1.98.

## Be Prepared 3.7

Solve $3.5=0.7n.3.5=0.7n.$
If you missed this problem, review Example 2.13.

## Be Prepared 3.8

Subtract $50−37.45.50−37.45.$
If you missed this problem, review Example 1.26.

## Translate and Solve Basic Percent Equations

We will solve percent equations using the methods we used to solve equations with fractions or decimals. Without the tools of algebra, the best method available to solve percent problems was by setting them up as proportions. Now as an algebra student, you can just translate English sentences into algebraic equations and then solve the equations.

We can use any letter you like as a variable, but it is a good idea to choose a letter that will remind us of what you are looking for. We must be sure to change the given percent to a decimal when we put it in the equation.

## Example 3.12

Translate and solve: What number is 35% of 90?

## Try It 3.23

Translate and solve:

What number is 45% of 80?

## Try It 3.24

Translate and solve:

What number is 55% of 60?

We must be very careful when we translate the words in the next example. The unknown quantity will not be isolated at first, like it was in Example 3.12. We will again use direct translation to write the equation.

## Try It 3.26

Translate and solve:

## Example 3.16

The label on Masao’s breakfast cereal said that one serving of cereal provides 85 milligrams (mg) of potassium, which is 2% of the recommended daily amount. What is the total recommended daily amount of potassium?

## Try It 3.31

One serving of wheat square cereal has seven grams of fiber, which is 28% of the recommended daily amount. What is the total recommended daily amount of fiber?

## Try It 3.32

One serving of rice cereal has 190 mg of sodium, which is 8% of the recommended daily amount. What is the total recommended daily amount of sodium?

## Example 3.17

Mitzi received some gourmet brownies as a gift. The wrapper said each brownie was 480 calories, and had 240 calories of fat. What percent of the total calories in each brownie comes from fat?

## Try It 3.33

Solve. Round to the nearest whole percent.

Veronica is planning to make muffins from a mix. The package says each muffin will be 230 calories and 60 calories will be from fat. What percent of the total calories is from fat?

## Try It 3.34

Solve. Round to the nearest whole percent.

The mix Ricardo plans to use to make brownies says that each brownie will be 190 calories, and 76 calories are from fat. What percent of the total calories are from fat?

## Find Percent Increase and Percent Decrease

People in the media often talk about how much an amount has increased or decreased over a certain period of time. They usually express this increase or decrease as a percent.

To find the percent increase, first we find the amount of increase, the difference of the new amount and the original amount. Then we find what percent the amount of increase is of the original amount.

## How To

### Find the Percent Increase.

1. Step 1. Find the amount of increase.
$new amount−original amount=increasenew amount−original amount=increase$
2. Step 2. Find the percent increase.
The increase is what percent of the original amount?

## Example 3.18

In 2011, the California governor proposed raising community college fees from $26 a unit to$36 a unit. Find the percent increase. (Round to the nearest tenth of a percent.)

## Try It 3.35

Find the percent increase. (Round to the nearest tenth of a percent.)

In 2011, the IRS increased the deductible mileage cost to 55.5 cents from 51 cents.

## Try It 3.36

Find the percent increase.

In 1995, the standard bus fare in Chicago was $1.50. In 2008, the standard bus fare was$2.25.

Finding the percent decrease is very similar to finding the percent increase, but now the amount of decrease is the difference of the original amount and the new amount. Then we find what percent the amount of decrease is of the original amount.

## How To

### Find the Percent Decrease.

1. Step 1. Find the amount of decrease.
$original amount−new amount=decreaseoriginal amount−new amount=decrease$
2. Step 2. Find the percent decrease.
Decrease is what percent of the original amount?

## Example 3.19

The average price of a gallon of gas in one city in June 2014 was $3.71. The average price in that city in July was$3.64. Find the percent decrease.

## Try It 3.37

Find the percent decrease. (Round to the nearest tenth of a percent.)

The population of North Dakota was about 672,000 in 2010. The population is projected to be about 630,000 in 2020.

## Try It 3.38

Find the percent decrease.

Last year, Sheila’s salary was $42,000. Because of furlough days, this year, her salary was$37,800.

## Solve Simple Interest Applications

Do you know that banks pay you to keep your money? The money a customer puts in the bank is called the principal, P, and the money the bank pays the customer is called the interest. The interest is computed as a certain percent of the principal; called the rate of interest, r. We usually express rate of interest as a percent per year, and we calculate it by using the decimal equivalent of the percent. The variable t, (for time) represents the number of years the money is in the account.

To find the interest we use the simple interest formula, $I=Prt.I=Prt.$

## Simple Interest

If an amount of money, P, called the principal, is invested for a period of t years at an annual interest rate r, the amount of interest, I, earned is

$I=PrtwhereI=interestP=principalr=ratet=timeI=PrtwhereI=interestP=principalr=ratet=time$

Interest earned according to this formula is called simple interest.

Interest may also be calculated another way, called compound interest. This type of interest will be covered in later math classes.

The formula we use to calculate simple interest is $I=Prt.I=Prt.$ To use the formula, we substitute in the values the problem gives us for the variables, and then solve for the unknown variable. It may be helpful to organize the information in a chart.

Susana invested a principal of $36,000 in her bank account with interest rate 6.5%. How much interest did she earn in 3 years? There may be times when we know the amount of interest earned on a given principal over a certain length of time, but we don’t know the rate. To find the rate, we use the simple interest formula, substitute in the given values for the principal and time, and then solve for the rate. ## Example 3.21 Loren loaned his brother$3,000 to help him buy a car. In 4 years his brother paid him back the $3,000 plus$660 in interest. What was the rate of interest?

$I=660P=3,000r=?t=4yearsI=660P=3,000r=?t=4years$

Notice that in this example, Loren’s brother paid Loren interest, just like a bank would have paid interest if Loren invested his money there.

## Try It 3.45

Find the amount of discount and the sale price:

Sergio bought a belt that was discounted 40% from an original price of $29. ## Try It 3.46 Find the amount of discount and the sale price: Oscar bought a barbecue that was discounted 65% from an original price of$395.

There may be times when we know the original price and the sale price, and we want to know the discount rate. To find the discount rate, first we will find the amount of discount and then use it to compute the rate as a percent of the original price. Example 3.24 will show this case.

## Example 3.24

Jeannette bought a swimsuit at a sale price of $13.95. The original price of the swimsuit was$31. Find the amount of discount and discount rate.

## Try It 3.47

Find the amount of discount and the discount rate.

Lena bought a kitchen table at the sale price of $375.20. The original price of the table was$560.

## Try It 3.48

Find the amount of discount and the discount rate.

Nick bought a multi-room air conditioner at a sale price of $340. The original price of the air conditioner was$400.

Applications of mark-up are very common in retail settings. The price a retailer pays for an item is called the original cost. The retailer then adds a mark-up to the original cost to get the list price, the price he sells the item for. The mark-up is usually calculated as a percent of the original cost. To determine the amount of mark-up, multiply the mark-up rate by the original cost.

We summarize the mark-up model in the box below.

## Mark-Up

$amount of mark-up=mark-up rate×original costlist price=original cost+amount of mark upamount of mark-up=mark-up rate×original costlist price=original cost+amount of mark up$

Keep in mind that the list price should always be more than the original cost.

## Try It 3.50

Find the amount of mark-up and the list price.

The Auto Resale Store bought Pablo’s Toyota for $8,500. They marked the price up 35%. ## Section 3.2 Exercises ### Practice Makes Perfect Translate and Solve Basic Percent Equations In the following exercises, translate and solve. 67. What number is 45% of 120? 68. What number is 65% of 100? 69. What number is 24% of 112? 70. What number is 36% of 124? 71. 250% of 65 is what number? 72. 150% of 90 is what number? 73. 800% of 2250 is what number? 74. 600% of 1740 is what number? 75. 28 is 25% of what number? 76. 36 is 25% of what number? 77. 81 is 75% of what number? 78. 93 is 75% of what number? 79. 8.2% of what number is$2.87?

80.

6.4% of what number is $2.88? 81. 11.5% of what number is$108.10?

82.

12.3% of what number is $92.25? 83. What percent of 260 is 78? 84. What percent of 215 is 86? 85. What percent of 1500 is 540? 86. What percent of 1800 is 846? 87. 30 is what percent of 20? 88. 50 is what percent of 40? 89. 840 is what percent of 480? 90. 790 is what percent of 395? Solve Percent Applications In the following exercises, solve. 91. Geneva treated her parents to dinner at their favorite restaurant. The bill was$74.25. Geneva wants to leave 16% of the total bill as a tip. How much should the tip be?

92.

When Hiro and his co-workers had lunch at a restaurant near their work, the bill was $90.50. They want to leave 18% of the total bill as a tip. How much should the tip be? 93. Trong has 12% of each paycheck automatically deposited to his savings account. His last paycheck was$2165. How much money was deposited to Trong’s savings account?

94.

Cherise deposits 8% of each paycheck into her retirement account. Her last paycheck was $1,485. How much did Cherise deposit into her retirement account? 95. One serving of oatmeal has eight grams of fiber, which is 33% of the recommended daily amount. What is the total recommended daily amount of fiber? 96. One serving of trail mix has 67 grams of carbohydrates, which is 22% of the recommended daily amount. What is the total recommended daily amount of carbohydrates? 97. A bacon cheeseburger at a popular fast food restaurant contains 2070 milligrams (mg) of sodium, which is 86% of the recommended daily amount. What is the total recommended daily amount of sodium? 98. A grilled chicken salad at a popular fast food restaurant contains 650 milligrams (mg) of sodium, which is 27% of the recommended daily amount. What is the total recommended daily amount of sodium? 99. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa’s original weight? 100. Tricia got a 6% raise on her weekly salary. The raise was$30 per week. What was her original salary?

101.

Yuki bought a dress on sale for $72. The sale price was 60% of the original price. What was the original price of the dress? 102. Kim bought a pair of shoes on sale for$40.50. The sale price was 45% of the original price. What was the original price of the shoes?

103.

Tim left a $9 tip for a$50 restaurant bill. What percent tip did he leave?

104.

Rashid left a $15 tip for a$75 restaurant bill. What percent tip did he leave?

105.

The nutrition fact sheet at a fast food restaurant says the fish sandwich has 380 calories, and 171 calories are from fat. What percent of the total calories is from fat?

106.

The nutrition fact sheet at a fast food restaurant says a small portion of chicken nuggets has 190 calories, and 114 calories are from fat. What percent of the total calories is from fat?

107.

Emma gets paid $3,000 per month. She pays$750 a month for rent. What percent of her monthly pay goes to rent?

108.

Dimple gets paid $3,200 per month. She pays$960 a month for rent. What percent of her monthly pay goes to rent?

Find Percent Increase and Percent Decrease

In the following exercises, solve.

109.

Tamanika got a raise in her hourly pay, from $15.50 to$17.36. Find the percent increase.

110.

Ayodele got a raise in her hourly pay, from $24.50 to$25.48. Find the percent increase.

111.

Annual student fees at the University of California rose from about $4,000 in 2000 to about$12,000 in 2010. Find the percent increase.

112.

The price of a share of one stock rose from $12.50 to$50. Find the percent increase.

113.

According to Time magazine annual global seafood consumption rose from 22 pounds per person in the 1960s to 38 pounds per person in 2011. Find the percent increase. (Round to the nearest tenth of a percent.)

114.

In one month, the median home price in the Northeast rose from $225,400 to$241,500. Find the percent increase. (Round to the nearest tenth of a percent.)

115.

A grocery store reduced the price of a loaf of bread from $2.80 to$2.73. Find the percent decrease.

116.

The price of a share of one stock fell from $8.75 to$8.54. Find the percent decrease.

117.

Hernando’s salary was $49,500 last year. This year his salary was cut to$44,055. Find the percent decrease.

118.

In 10 years, the population of Detroit fell from 950,000 to about 712,500. Find the percent decrease.

119.

In 1 month, the median home price in the West fell from $203,400 to$192,300. Find the percent decrease. (Round to the nearest tenth of a percent.)

120.

Sales of video games and consoles fell from $1,150 million to$1,030 million in 1 year. Find the percent decrease. (Round to the nearest tenth of a percent.)

Solve Simple Interest Applications

In the following exercises, solve.

121.

Casey deposited $1,450 in a bank account with interest rate 4%. How much interest was earned in two years? 122. Terrence deposited$5,720 in a bank account with interest rate 6%. How much interest was earned in 4 years?

123.

Robin deposited $31,000 in a bank account with interest rate 5.2%. How much interest was earned in 3 years? 124. Carleen deposited$16,400 in a bank account with interest rate 3.9%. How much interest was earned in 8 years?

125.

Hilaria borrowed $8,000 from her grandfather to pay for college. Five years later, she paid him back the$8,000, plus $1,200 interest. What was the rate of interest? 126. Kenneth loaned his niece$1,200 to buy a computer. Two years later, she paid him back the $1,200, plus$96 interest. What was the rate of interest?

127.

Lebron loaned his daughter $20,000 to help her buy a condominium. When she sold the condominium four years later, she paid him the$20,000, plus $3,000 interest. What was the rate of interest? 128. Pablo borrowed$50,000 to start a business. Three years later, he repaid the $50,000, plus$9,375 interest. What was the rate of interest?

129.

In 10 years, a bank account that paid 5.25% earned $18,375 interest. What was the principal of the account? 130. In 25 years, a bond that paid 4.75% earned$2,375 interest. What was the principal of the bond?

131.

Joshua’s computer loan statement said he would pay $1,244.34 in interest for a 3-year loan at 12.4%. How much did Joshua borrow to buy the computer? 132. Margaret’s car loan statement said she would pay$7,683.20 in interest for a 5-year loan at 9.8%. How much did Margaret borrow to buy the car?

Solve Applications with Discount or Mark-up

In the following exercises, find the sale price.

133.

Perla bought a cell phone that was on sale for $50 off. The original price of the cell phone was$189.

134.

Sophie saw a dress she liked on sale for $15 off. The original price of the dress was$96.

135.

Rick wants to buy a tool set with original price $165. Next week the tool set will be on sale for$40 off.

136.

Angelo’s store is having a sale on televisions. One television, with original price $859, is selling for$125 off.

In the following exercises, find the amount of discount and the sale price.

137.

Janelle bought a beach chair on sale at 60% off. The original price was $44.95. 138. Errol bought a skateboard helmet on sale at 40% off. The original price was$49.95.

139.

Kathy wants to buy a camera that lists for $389. The camera is on sale with a 33% discount. 140. Colleen bought a suit that was discounted 25% from an original price of$245.

141.

Erys bought a treadmill on sale at 35% off. The original price was $949.95 (round to the nearest cent.) 142. Jay bought a guitar on sale at 45% off. The original price was$514.75 (round to the nearest cent.)

In the following exercises, find the amount of discount and the discount rate. (Round to the nearest tenth of a percent if needed.)

143.

Larry and Donna bought a sofa at the sale price of $1,344. The original price of the sofa was$1,920.

144.

Hiroshi bought a lawnmower at the sale price of $240. The original price of the lawnmower is$300.

145.

Patty bought a baby stroller on sale for $301.75. The original price of the stroller was$355.

146.

Bill found a book he wanted on sale for $20.80. The original price of the book was$32.

147.

Nikki bought a patio set on sale for $480. The original price was$850. To the nearest tenth of a percent, what was the rate of discount?

148.

Stella bought a dinette set on sale for $725. The original price was$1,299. To the nearest tenth of a percent, what was the rate of discount?

In the following exercises, find the amount of the mark-up and the list price.

149.

Daria bought a bracelet at original cost $16 to sell in her handicraft store. She marked the price up 45%. 150. Regina bought a handmade quilt at original cost$120 to sell in her quilt store. She marked the price up 55%.

151.

Tom paid $0.60 a pound for tomatoes to sell at his produce store. He added a 33% mark-up. 152. Flora paid her supplier$0.74 a stem for roses to sell at her flower shop. She added an 85% mark-up.

153.

Alan bought a used bicycle for $115. After re-conditioning it, he added 225% mark-up and then advertised it for sale. 154. Michael bought a classic car for$8,500. He restored it, then added 150% mark-up before advertising it for sale.

### Everyday Math

155.

Leaving a Tip At the campus coffee cart, a medium coffee costs $1.65. MaryAnne brings$2.00 with her when she buys a cup of coffee and leaves the change as a tip. What percent tip does she leave?

156.

Splitting a Bill Four friends went out to lunch and the bill came to $53.75. They decided to add enough tip to make a total of$64, so that they could easily split the bill evenly among themselves. What percent tip did they leave?

### Writing Exercises

157.

Without solving the problem “44 is 80% of what number” think about what the solution might be. Should it be a number that is greater than 44 or less than 44? Explain your reasoning.

158.

Without solving the problem “What is 20% of 300?” think about what the solution might be. Should it be a number that is greater than 300 or less than 300? Explain your reasoning.

159.

After returning from vacation, Alex said he should have packed 50% fewer shorts and 200% more shirts. Explain what Alex meant.

160.

Because of road construction in one city, commuters were advised to plan that their Monday morning commute would take 150% of their usual commuting time. Explain what this means.

### Self Check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

After reviewing this checklist, what will you do to become confident for all goals?