Back To Course

GED Math: Quantitative, Arithmetic & Algebraic Problem Solving10 chapters | 73 lessons | 7 flashcard sets

Instructor:
*Jeff Calareso*

Jeff teaches high school English, math and other subjects. He has a master's degree in writing and literature.

In this lesson, we'll learn how simple interest can earn (or cost) you money. We'll also look at percent change problems to better understand how discounts and markups work.

Few things you'll learn in math are as useful as operations with percents. Once you understand concepts like simple interest and percent change, you'll have knowledge that can literally put money in your pocket.

**Simple interest** is just what it sounds like: an easy way of calculating how much interest you can earn (or owe). Let's say you put some money in the bank and it's earning interest. The money you start with is your principal. The word 'principal' also means first, so this money is where your investment starts. Simple interest is a way of calculating just how much money your principal is earning you.

**Percent change** is closely related. Every time you see a sale advertising '30% off!' or if you want to leave a 20% tip at a restaurant, you're dealing with percent change. As with simple interest, you start with a principal amount. The percent change is the amount that principal increases or decreases. Let's look at examples of both of these ideas.

Let's start with simple interest. What if you deposit $500 and you'll earn 4% interest over 3 years. To calculate how much money your principal investment will earn you, we need a formula: ** I = prt**.

*I* is for interest, of course. That's what we want to find. To get it, we just need to know three things: what we start with (the principal, or *p*), the interest rate (*r*) and the time (*t*) the money needs to sit.

Here, the principal is $500, the rate is 4% (or .04) and the time is 3 years. If we plug that into *I* = *prt*, we get *I* = (500)(.04)(3). That gets us $60. So you can earn $60 just by letting your money sit around for 3 years.

Sometimes, simple interest doesn't get you money, it costs you money. Imagine you borrow $800 from your friend Steve to help you buy a life-size statue of Bigfoot. You'd use your own money, but remember that it's locked away earning that 4% interest. This statue is a weird investment, but it's great of Steve to help. Well, not that great. You promise to pay Steve back in 2 years, but he'll charge you 7% interest.

To find out how much you'll need to pay Steve back, use the same formula: *I* = *prt*. This time, the principal is $800, the interest rate is 7% (or .07) and the time is 2 years. So that's *I* = (800)(.07)(2). That gives us $112.

But that's not what you owe Steve. If you recall, the formula you just used was to find the *interest.* You owe Steve the $112 interest plus the original $800. So the amount you'll have to pay back is $800 + $112, or $912. But, you know, life-size Bigfoot statue!

You're expecting your Bigfoot statue to go up in value. The guy who sold it to you said it should be worth 30% more than you paid for it. That's great! But how much is that in cold, hard cash? You paid $800 of your moneyâ€¦ well, Steve's money. What is 30% more than $800?

This is a percent change problem. Since we want to find out what 30% more than our principal is, we're essentially figuring out what 130% of $800 is. (Remember, 100% of $800 is just $800.) 130% can be written as 1.30. To find the projected value of your statue, we multiply: $800 * 1.3. This gets us $1040. That's a nice increase!

Note that this is the same way you'd calculate a tip. If a check comes for $32 and you want to leave a 20% tip, just multiply $32 * 1.20 to get $38.40. That's the principal plus the tip.

As for your statue, the weeks go by and no one wants to buy it. And Steve really wants his money back. It turns out flipping Bigfoot statues for profit isn't a good business. You finally decide to offer it for 25% off your $1040 asking price. What will this be?

You could multiply $1040 by .25. That gets you $260. So you're offering $260 off. $1040 - $260 is $780. Or, you could multiply $1040 by .75. In other words, just like 130% is 30% over 100%, 75% is 100% minus 25%. $1040 * .75 = $780. Well, that's not going to help you pay back Steve.

So let's try one more thing. You need to pay Steve $912. You were asking $1040 for the statue. What's the percent off you could offer to get you $912?

To find the percent change, use this formula:

Our new value is $912. Our old value is $1040. 912 - 1040 = -128. We divide that by the absolute value of 1040. -128/1040 = -0.12. Finally, we multiply that by 100%: -0.12 * 100% = -12%. The negative sign means the new value is 12% less than the old value.

It's not as impressive sounding as 25% off, but it'll help you pay off Steve.

In summary, we learned about simple interest and percent change. **Simple interest** is how you calculate how much interest a sum of money will accumulate. The formula is ** I = prt**, or interest = (principal)(rate)(time). We also looked at

To unlock this lesson you must be a Study.com Member.

Create your account

Already a member? Log In

BackDid you know… We have over 160 college courses that prepare you to earn credit by exam that is accepted by over 2,000 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

You are viewing lesson
Lesson
3 in chapter 5 of the course:

Back To Course

GED Math: Quantitative, Arithmetic & Algebraic Problem Solving10 chapters | 73 lessons | 7 flashcard sets

- MTLE Pedagogy - Secondary (Grades 5-12): Study Guide & Practice
- MTLE Pedagogy - Elementary (Grades K-6): Study Guide & Practice
- Computer Science 113: Programming in Python
- CHSPE Information Guide
- DSST Information Guide
- Required Assignments for Communications 301
- Required Assignments for Psychology 306
- Computing with Numbers
- Introduction to Python Programming
- Object-Oriented Programming
- Study.com CHSPE Scholarship: Application Form & Information
- TExES Exams & Certification Areas
- CSET Math Pass Rates
- TEAS Testing Sites In California
- Study.com DSST Scholarship: Application Form & Information
- How to Pass the CSET Multiple Subject Test
- How to Study for the MTTC Tests

- Reputation in Othello
- Law of Mass Action: Definition, Application & Equation
- Student Self-Assessment & Peer Assessment
- What is White Privilege? - Definition, Examples & Statistics
- Social Media Networks for Social Selling: Types, Pros & Cons
- Practical Application for C++ Programming: Loops
- Basics of Variables in C++ Programming
- Interconnection Networks: Role & Types
- Quiz & Worksheet - Menelaus Paris Duel in The Iliad
- Quiz & Worksheet - Beginnings of U.S. Free Enterprise System
- Quiz & Worksheet - History of Spain
- Quiz & Worksheet - Bradford Protein Assay Protocol
- Quiz & Worksheet - Uniform Production in Lean
- Flashcards - Introduction to Research Methods in Psychology
- Flashcards - Clinical Assessment in Psychology

- Prentice Hall World History Connections to Today Volume 1: Online Textbook Help
- MTTC Reading Specialist (092): Practice & Study Guide
- High School Psychology Textbook
- Real Estate Exam Prep
- Smarter Balanced Assessments - ELA Grades 3-5: Test Prep & Practice
- HiSET: Absolutism in Western Europe (1648-1715)
- Chapter 8: Exponents and Exponential Functions
- Quiz & Worksheet - Amphipathic Molecules
- Quiz & Worksheet - Writing with Idioms or Phrasal Verbs
- Quiz & Worksheet - George Whitefield and the Great Awakening
- Quiz & Worksheet - The Murder of Gonzago in Hamlet
- Quiz & Worksheet - Functions of Macronutrients

- Figurative Language in The Necklace
- Choosing the Correct Email Platform
- Washington State Science Standards for 2nd Grade
- Suicide Prevention Organizations & Groups
- Homeschooling in Canada
- States of Matter Lesson Plan
- How Does Homeschooling Work?
- Minnesota Science Standards
- Homeschooling in Louisiana
- How to Pass the Social Studies GED Test
- WIDA Can Do Descriptors for Kindergarten
- Nebraska State Science Standards

Browse by subject