1. **State the problem:** A student is at the edge of a canyon 260 m wide and hears her echo 1.5 s later. We need to find the speed of sound waves. 2. **Understand the echo:** The sound travels to the opposite side of the canyon and back, so the total distance traveled by the sound is twice the width of the canyon. 3. **Write the formula for speed:** Speed is distance divided by time, so $$\text{speed} = \frac{\text{distance}}{\text{time}}$$ 4. **Calculate the total distance:** $$\text{distance} = 2 \times 260 = 520 \text{ meters}$$ 5. **Calculate the speed:** $$\text{speed} = \frac{520}{1.5}$$ 6. **Simplify the fraction:** $$\text{speed} = \frac{\cancel{520}}{\cancel{1.5}} = 346.67 \text{ m/s (approx)}$$ 7. **Final answer:** The speed of sound waves is approximately **347 m/s**, which is close to the given value of $3.5 \times 10^{2}$ m/s.