Apply the problem-solving strategy to find the rate of simple interest given the principal, interest earned, and time.

Problem: Loren lent 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 simple interest?

1. Read the problem carefully.
2. Identify what to find: the rate of simple interest.
3. Name the unknown: Let $r$ = rate of interest.
4. Translate into an equation using the simple interest formula $I = Prt$. Given $I = 660$, $P = 3{,}000$, and $t = 4$: $660 = (3{,}000)r(4)$
5. Solve the equation: Multiply $3{,}000$ and $4$: $660 = 12{,}000r$ Divide both sides by $12{,}000$: $0.055 = r$ Change the decimal to percent form: $5.5\% = r$
6. Check the result: $I = Prt$ $660 \stackrel{?}{=} (3{,}000)(0.055)(4)$ $660 = 660 \checkmark$
7. Answer: The rate of interest was $5.5\%$.