A brother lends his sister $5,000. Three years later she repays the $5,000 plus $900 in interest. What annual rate of simple interest did she pay?

1. Use the simple interest formula I = Prt with I = 900 dollars, P = 5,000 dollars, and t = 3 years. 2. Substitute: 900 = 5,000 x r x 3. 3. Multiply the knowns: 900 = 15,000r. 4. Divide both sides by 15,000: r = 900 / 15,000 = 0.06. 5. Write as a percent: 6.0% per year. 6. 18.0% is the trap of dividing 900 by 5,000 only, which ignores the 3 years entirely. 7. 5.4% and 6.6% are near misses that fail the check: 5,000 x 0.06 x 3 = 900 dollars exactly. _Source: OpenStax Prealgebra (CC BY 4.0), Ch 6 "Percents", section 6.4 Solve Simple Interest Applications_