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1. Credit and Debt HS
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Chapter 2
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Credit and Debt HS

This lesson will teach you the theory required to fully understand the topic and will present both exercises and selftests to gauge your understanding.
Problem Solving Reasoning and Communication Error Analysis Modeling Using Tools Precision Pattern Recognition
Lesson Settings & Tools
12 Theory slides
15 Exercises - Grade E - A
Each lesson is meant to take 1-2 classroom sessions
Credit and Debt HS
Slide of 12

Exercise

Use the Distributive Property to rewrite the expression. 3(-2 x+4)=

Answer

- 6x+12

Hint

Be careful with negatives when distributing.

Solution

We are asked to rewrite the given expression using the Distributive Property. To do so, we need to distribute 3 to the terms inside the parentheses.
3(-2 x+4)
3(-2 x) + 3(4)
- 3(2x) + 3(4)
- 6x+12

Form

Discussion

Assets and Liabilities

Use the Distributive Property to rewrite the expression. 4(x+9)

Answer

4x+36

Hint

The Distributive Property states that to multiply a sum or a difference by a number, multiply each term inside the parentheses by the number outside the parentheses.

Solution

We are asked to rewrite the given expression using the Distributive Property. To do so, we need to distribute 4 to the terms inside the parentheses.
4(x+9)
4(x) + 4(9)
4x+36

Form

Explore

Asset or liability?

Sort the items into assets and liabilities. Remember, assets are things that you own that have value and liabilities are things you owe!

Discussion

What Is a Loan?

Describe the relationship between the terms in each arithmetic sequence. Then write the next three terms in sequence. 4,11,18,25, ...

Answer

Pattern: 7 is added to the previous term
Next Three Terms: 32, 39, 46

Hint

What is the change between pairs of consecutive terms?

Solution

By observing the change that occurs between each consecutive term, we can describe the pattern of the sequence. Here, we see that the common difference from one term to the next is adding 7. 4 +7 →11 +7 →18 +7 →25 To find the next three terms in the sequence, we will extend this pattern three times. 4 +7 →11 +7 →18 +7 →25 +7 → 32 +7 → 39 +7 → 46

Form

Discussion

Interest Rate

The interest rate is the percentage of the principal a lender charges for borrowing money. It tells you how much the loan will cost over time. The simplest way interest can be calculated is through simple interest. Simple interest is calculated only on the principal amount. There is a formula for finding simple interest.


I=Prt

The variables used in the formula are defined as:

  • I is the interest earned
  • P is the principal amount
  • r is the annual interest rate in decimal form
  • t is the time spent in years
Discussion

What Is Compound Interest and How Does It Work?

Compound interest is interest that is calculated not just on the original loan or deposit — the principal — but also on the interest already added. This concept is often referred to as interest on interest. In contrast, simple interest is calculated only on the principal amount. Compound interest LCstyle.jpg There is a formula for finding how compound interest affects the balance of a savings account or loan.


A = P(1 + r/n)^(nt)

The variables used in the formula are defined as:

  • A is the amount of money accumulated after t years, including interest
  • P is the principal amount
  • r is the annual interest rate in decimal form
  • n is the number of times that interest is compounded per year
  • t is the time spent paying back the loan in years
{"codehash":"fb9b0059182883683e2a3b7ea4227a63"}
Discussion

Simple and Compound Interest Calculator

Use this calculator to find simple or compound interest. Enter the required parameters for your specific case and choose the interest type to get the result.

Theory

Ethan's Credit Card

Ethan buys a gaming laptop for $1000 using a credit card with a 20 % annual interest rate, compounded monthly. He was offered a deal where he doesn't have to make any payments for the first year, so he doesn't. Ethans credit card LCstyle.jpg

a How much will Ethan owe after 1 year? Round to the nearest whole dollar amount.
b How much interest did Ethan pay?
c What could Ethan have done to avoid paying any compound interest?
d Now imagine that Ethan only used the credit card because he wanted to earn points on the card. He pays off his entire balance before the first bill arrives. How much interest does he pay?

Hint

b Subtract the original amount he borrowed from the total he owes after one year.
c Think about the fact that the interest compounds each month. What if he pays off the balance in fewer months?
d The interest accrues after each month passes. What happens if there is no debt remaining at the end of the first month?

Solution

a Ethan's initial balance is $1000 and the annual interest rate is 20 %, compounded monthly. We calculate the total amount owed after 1 year, where the interest compounds a total of 12 times, using the compound interest formula.
A = P (1 + rn)^(nt) Remember that, in this formula, P is the principal, r is the annual interest rate written as a decimal, n is the compounding periods per year, and t is the time in years.
A = P (1 + r/n)^(nt)
A = 1000 (1 + 0.20/12)^(12 * 1)
A = 1000 (1 + 0.016667)^(12)
A = 1000 (1.016667)^(12)
A ≈ 1219.40
A ≈ 1219
After 1 year, Ethan will owe approximately $1219.
b The total interest paid is the difference between the total amount paid and the principal.

$1219 - $1000 = $219 Ethan paid $219 in interest.

c Let's consider the given options to determine what Ethan could have done to avoid paying any compound interest.
Option Logic Conclusion
Pay off the full balance right away. No balance remains to accrue interest if paid immediately. No interest will be paid ✓
Buy a cheaper laptop. A lower purchase amount reduces total interest but doesn’t prevent it from accruing. Less interest would be paid overall *
Pay the balance off in equal payments over the course of the year. Interest accrues from the first month, but consistent payments reduce the balance faster. Less interest would be paid overall *
Negotiate with the credit card company for better terms and conditions. A successful negotiation might lower the interest rate but won't guarantee zero interest. Less interest might be paid *
d If Ethan pays the full balance before the first bill arrives, no interest is charged. Ethan will pay $0 in interest.

Teenager-bill-balance.png

Exercise

An ounce is 116 of a pound. How many ounces are in 8 34 pounds?

Answer

140oz

Hint

To divide by a fraction, multiply by its reciprocal.

Solution

We are asked to find how many ounces there are in 8 34 pounds. We know that an ounce is 116 of a pound. Let's write an expression that represents the number of ounces in 8 34 pounds. 8 34 ÷ 1/16 Now we can evaluate the expression. We will start by writing the mixed number as an improper fraction.
8 34 ÷ 1/16
8* 4 + 3/4 ÷ 1/16
32 + 3/4 ÷ 1/16
35/4 ÷ 1/16
Next, we will divide the fractions. Recall that to divide by a fraction, we need to multiply by its reciprocal.
35/4 ÷ 1/16
35/4 * 16/1
35* 16/4* 1
560/4
140
We got that there are 140 ounces in 8 34 pounds.

Form

Theory

Layla's Car Loan

Layla takes out a $10 000 loan to buy her first car. She is told that the interest rate is 5 %, compounded monthly, and she plans to pay it off in 5 years. She uses this number to build her budget. After reviewing her contract, she learns that there are extra fees included in the loan, which brings the annual percentage rate (APR) to 6 %.

Laylas car loan LCstyle.jpg

a How much did Layla think her total repayment would be (based on 5 % interest)? Round to the nearest whole dollar amount.
b What is the actual total cost of the loan (based on 6 % APR compounded monthly)? Round to the nearest whole dollar amount.
c What is the difference between what she expected to pay and what she actually has to pay?

Hint

a Use 5 % as the interest rate in the compound interest formula.
b Use 6 % as the interest rate now.
c Subtract the expected total from the actual total.

Solution

a We want to calculate the total repayment Layla expected for her $10 000 loan based on a 5 % interest rate compounded monthly — which means that it happens 12 times every year — for 5 years. Let's recall the formula for compound interest.
A = P (1 + rn)^(nt) In this formula, P is the principal, r is the annual interest rate in decimal form, n is the number of compounding periods per year, and t is the number of years.
A = P (1 + r/n)^(nt)
A = 10 000 (1 + 0.05/12)^(12 * 5)
A = 10 000 (1 + 0.004167)^(60)
A = 10 000 (1.004167)^(60)
A = 12 833.842394...
A ≈ 12 834
Layla expected to repay $12 834 on her loan.
b Now we calculate the actual repayment using the APR of 6 % compounded monthly. Everything about the compound interest formula will remain the same except that r is now 0.06.
A = 10 000 (1 + 0.06/12)^(12* 5)
Evaluate right-hand side
A = 10 000 (1 + 0.005)^(60)
A = 10 000 (1.005)^(60)
A = 13 488.501525...
A ≈ 13 489
The actual total cost of the loan over 5 years is $13 489.
c We subtract the expected repayment from the actual repayment to find the difference.

$13 489 - $12 834 = $655 Layla will have to pay $655 more than she expected.

Exercise

Lucas babysat his younger sister 2 12 hours on Friday and 3 23 hours on Saturday. How much longer did Lucas babysit his younger sister on Saturday than on Friday?

Answer

1 16 hours

Hint

To subtract two mixed numbers, start by writing each mixed number as an improper fraction.

Solution

We are given that Lucas babysat his younger sister for 2 12 hours on Friday and for 3 23 hours on Saturday. We are asked to find how much longer Lucas babysat his younger sister on Saturday than on Friday. Let's write an expression that represents the desired difference. 3 23-2 12 Now we will evaluate the expression. We will start by writing each mixed number as an improper fraction.
3 23-2 12
3* 3 + 2/3-2* 2 + 1/2
9 + 2/3-4 + 1/2
11/3-5/2
Next, we can subtract the fractions. Recall that to subtract fractions with different denominators, we follow three steps.
  1. Rename the fractions using the least common denominator (LCD).
  2. Subtract as with like fractions.
  3. If necessary, simplify the difference.
Let's start by finding the least common denominator. The least common denominator of two fractions is the least common multiple of the denominators. 11/3, 5/2 In this case, the denominators are both prime numbers. This means that the least common multiple of 3 and 2 is equal to 3 * 2 or 6. Let's move on to finding the equivalent fractions with a denominator of 6. To do so, we need to multiply the numerator and denominator of 113 by 2 and the numerator and denominator of 52 by 3.
11/3-5/2
11* 2/3* 2-5/2
22/6-5/2
22/6-5* 3/2* 3
22/6-15/6
Now we can subtract the like fractions by subtracting the numerators and writing the result over the denominator.
22/6-15/6
22-15/6
7/6
Finally, we will write the improper fraction as a mixed number. We can do this by expressing the numerator as a sum of two positive integers where one of these integers will be a multiple of the denominator. In this case, we can write 7 as a sum of 6 and 1. 7/6=6+1/6 Then we will write the fraction as a sum of two fractions and simplify it to get a mixed number.
6+1/6
6/6+1/6
1+1/6
1 16
We got that on Saturday, Lucas babysat his younger sister for 1 16 hours longer than on Friday.

Form

Credit and Debt HS
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