1. Lesson 7.1.5
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2. Interest Formula Interest = Principal ($) • Rate(%) • Time (in years)
Many people refer to the formula as I = PRT
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3. GUIDED PRACTICE #1 YOU TRY #1
Step 1: Write down the formula.
Step 2: Underline the principal amount in red.
Step 3: Underline the rate amount in blue.
Step 4: Underline the time given in green.
Step 5: Substitute the values in place of the
variable it represents.
Step 6: Calculate the operations.
YOU TRY #1
Step 1: Write down the formula.
Step 2: Underline the principal amount in red.
Step 3: Underline the rate amount in blue.
Step 4: Underline the time given in green.
Step 5: Substitute the values in place of the
variable it represents.
Step 6: Calculate the operations.
Cheryl deposits $7,500 into the bank. She cannot withdraw or deposit any more money for 6 years. She earns 6% interest during that time.
How much money will she have earned at the end of 6 years?
What will her new balance be when at the end of the 6 years?
Craig just purchased a new sports car. He financed $45,000 and must pay it back over 5 years with 11% interest.
How much money will Craig have paid in interest by the time his car is paid off?
How much will he have paid in all for his car, including the interest after 5 years?
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4. GUIDED PRACTICE #2 YOU TRY #2 Step 1: Write down the formula.
Step 2: Underline the principal amount in red.
Step 3: Underline the rate amount in blue.
Step 4: Underline the time given in green.
Step 5: Substitute the values in place of the
variable it represents.
Step 6: Calculate the operations.
YOU TRY #2
Step 1: Write down the formula.
Step 2: Underline the principal amount in red.
Step 3: Underline the rate amount in blue.
Step 4: Underline the time given in green.
Step 5: Substitute the values in place of the
variable it represents.
Step 6: Calculate the operations.
Allison was left an inheritance from her Great Aunt. However, she is not able to touch it until she is 21 years old. Allison is 12 years old now, and inherited $50,000 that was put into an account that earns 2% interest.
How much interest will Allison have earned when she turns 21 years old?
If she decides to withdraw all of the money on her 21st birthday, how much money will she have?
Cameron has a balance of $10,200 on his credit card. He threw the card away so he can never use it again. He has years to pay off the balance. The interest rate on his card is 21%.
At the end of years, how much money in interest alone will Cameron have paid?
When Cameron finally pays off his credit card, how much will he have paid in all?
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5. GUIDED PRACTICE #3 YOU TRY #3 Step 1: Write down the formula.
Step 2: Underline the principal amount in red.
Step 3: Underline the rate amount in blue.
Step 4: Underline the time given in green.
Step 5: Substitute the values in place of the
variable it represents.
Step 6: Calculate the operations.
YOU TRY #3
Step 1: Write down the formula.
Step 2: Underline the principal amount in red.
Step 3: Underline the rate amount in blue.
Step 4: Underline the time given in green.
Step 5: Substitute the values in place of the
variable it represents.
Step 6: Calculate the operations.
Jocelyn was able to get a great interest rate on her house. She financed $150,000 for 15 years with an interest rate of 2.75%.
How much will Jocelyn pay in interest over 15 years?
By the time she pays off her mortgage, how much total will she have spent for her house?
Riley let her friend borrow $12,750. She wants to be paid back in years. She is going to charge her friend 5.5% in interest.
How much money in interest will Riley earn?
When Riley’s friend pays her back, how much money will she have gotten paid in all?
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6. Distance Formula
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7. If a car travels 400m in 20 seconds how fast is it going?
GUIDED PRACTICE #4
Step 1: Write down the formula.
Step 2: Substitute the values in place of the
variable it represents.
Step 3: Calculate the operations.
YOU TRY #4
Step 1: Write down the formula.
Step 2: Substitute the values in place of the
variable it represents.
Step 3: Calculate the operations.
If a car travels 400m in 20 seconds how fast is it going?
If you move 50 meters in 10 seconds, what is your speed?
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8. A plane travels 395,000 meters in 9000 seconds. What was its speed?
GUIDED PRACTICE #5
Step 1: Write down the formula.
Step 2: Substitute the values in place of the
variable it represents.
Step 3: Calculate the operations.
YOU TRY #5
Step 1: Write down the formula.
Step 2: Substitute the values in place of the
variable it represents.
Step 3: Calculate the operations.
You need to get to class, 200 meters away, and you can only walk in the hallways at about 1.5 m/s. (if you run any faster, you’ll be caught for running). How much time will it take to get to your class?
A plane travels 395,000 meters in 9000 seconds. What was its speed?
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9. GUIDED PRACTICE #6
Step 1: Write down the formula.
Step 2: Substitute the values in place of the
variable it represents.
Step 3: Calculate the operations.
YOU TRY #6
Step 1: Write down the formula.
Step 2: Substitute the values in place of the
variable it represents.
Step 3: Calculate the operations.
If you shout into the Grand Canyon, your voice travels at the speed of sound (340 m/s) to the bottom of the canyon and back, and you hear an echo. How deep is the Grand Canyon at a spot where you can hear your echo 5.2 seconds after you shout?
How far can your little brother get if he can travel at 2.5 m/s and in 5 seconds you will discover that his squirt gun has run out of paint?
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