1. **State the problem:** We are given that 1,700 British thermal units (Btu) are converted into useful work, and the total energy supplied is 4,300 Btu. We need to find the thermal efficiency as a percentage. 2. **Formula for thermal efficiency:** Thermal efficiency ($\eta$) is given by the ratio of useful work output to the energy input, multiplied by 100 to convert to a percentage: $$\eta = \frac{\text{Useful work output}}{\text{Energy input}} \times 100\%$$ 3. **Substitute the given values:** $$\eta = \frac{1700}{4300} \times 100\%$$ 4. **Calculate the fraction:** $$\frac{1700}{4300} = \frac{\cancel{1700}}{\cancel{4300}} = \frac{1700 \div 100}{4300 \div 100} = \frac{17}{43} \approx 0.3953$$ 5. **Calculate the efficiency percentage:** $$\eta \approx 0.3953 \times 100\% = 39.53\%$$ 6. **Round to the nearest whole percent:** $$\boxed{40\%}$$ **Final answer:** The thermal efficiency is approximately 40%.