Example 3.21 from Elementary Algebra, 3.2 Solve Percent Applications

Solve Percent Applications: 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?

\begin{matrix} I & = & $660 \\ P & = & $3,000 \\ r & = & ? \\ t & = & 4 years \end{matrix}

Work it out on paper first. Then open the solution one step at a time, and stop as soon as you can finish on your own.

Your turn

The book's Try It right after this example: same idea, new numbers. Only the answer is given.

Jim loaned his sister $5,000 to help her buy a house. In 3 years, she paid him the $5,000, plus $900 interest. What was the rate of interest?

How did it go?

Step 1. Read the problem.
Step 2. Identify what we are looking for.	the rate of interest
Step 3. Name what we are looking for. Choose a variable to represent that quantity.	Let $r =$ the rate of interest.
Step 4. Translate into an equation. Write the formula. Substitute in the given information.	$\begin{matrix} I = P r t \\ 660 = (3,000) r (4) \end{matrix}$
Step 5. Solve the equation. Divide. Change to percent form.	$\begin{matrix} 660 = (12,000) r \\ 0.055 = r \\ 5.5% = r \end{matrix}$
Step 6. Check: Does this make sense? $\begin{matrix} I = P r t \\ 660 \overset{?}{=} (3,000) (0.055) (4) \\ 660 = 660 ✓ \end{matrix}$
Step 7. Answer the question with a complete sentence.	The rate of interest was 5.5%.

Step 1. Read the problem.
Step 2. Identify what we are looking for.	the rate of interest
Step 3. Name what we are looking for. Choose a variable to represent that quantity.	Let $r =$ the rate of interest.
Step 4. Translate into an equation. Write the formula. Substitute in the given information.	$\begin{matrix} I = P r t \\ 660 = (3,000) r (4) \end{matrix}$
Step 5. Solve the equation. Divide. Change to percent form.	$\begin{matrix} 660 = (12,000) r \\ 0.055 = r \\ 5.5% = r \end{matrix}$
Step 6. Check: Does this make sense? $\begin{matrix} I = P r t \\ 660 \overset{?}{=} (3,000) (0.055) (4) \\ 660 = 660 ✓ \end{matrix}$
Step 7. Answer the question with a complete sentence.	The rate of interest was 5.5%.

Solve Percent Applications: example 3.21

Solution

Your turn

Solve Percent Applications: example 3.21

Solution

Your turn