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Example

Solving a Word Problem for the Rate of Simple Interest on Loren's Loan

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 44 years, his brother paid him back the $3{,}000 plus $660\$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 rr = rate of interest.
  4. Translate into an equation using the simple interest formula I=PrtI = Prt. Given I=660I = 660, P=3,000P = 3{,}000, and t=4t = 4: 660=(3,000)r(4)660 = (3{,}000)r(4)
  5. Solve the equation: Multiply 3{,}000 and 44: 660=12,000r660 = 12{,}000r Divide both sides by 12{,}000: 0.055=r0.055 = r Change the decimal to percent form: 5.5%=r5.5\% = r
  6. Check the result: I=PrtI = Prt 660 stackrel{?}{=} (3{,}000)(0.055)(4) 660 = 660 checkmark
  7. Answer: The rate of interest was 5.5%.

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Updated 2026-06-05

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