1. **State the problem:** Harrison lent his sister 3750 to buy a house. After 4 years, she paid back 3750 plus 600 interest. We need to find the rate of interest per year. 2. **Formula used:** The simple interest formula is $$I = P \times r \times t$$ where: - $I$ is the interest earned, - $P$ is the principal amount, - $r$ is the rate of interest per year (in decimal), - $t$ is the time in years. 3. **Identify known values:** - $I = 600$ - $P = 3750$ - $t = 4$ 4. **Substitute values into the formula:** $$600 = 3750 \times r \times 4$$ 5. **Simplify the equation:** $$600 = 15000 \times r$$ 6. **Solve for $r$:** $$r = \frac{600}{15000}$$ 7. **Show cancellation:** $$r = \frac{\cancel{600}}{\cancel{15000}} = \frac{2}{50} = 0.04$$ 8. **Convert to percentage:** $$r = 0.04 \times 100 = 4\%$$ **Final answer:** The rate of interest was 4%.