Doppler Effect for Moving Source Steps: - Use the Doppler formula for source moving away from stationary observer: $f' = f \frac{v}{v + v_s}$, where $f = 800$ Hz, $f' = 750$ Hz, $v = 340$ m/s. - Rearrange: $v_s = v \left( \frac{f}{f'} - 1 \right) = 340 \left( \frac{800}{750} - 1 \right) = 340 \left( \frac{16}{15} - 1 \right) = 340 \times \frac{1}{15} \approx 22.7$ m/s. - Approximate using $\frac{\Delta f}{f} \approx -\frac{v_s}{v}$: $v_s \approx -\frac{\Delta f}{f} v = \frac{50}{800} \times 340 = 21.25$ m/s away. - Options approximate to 21 m/s away, as exact 22.7 is closest to 21 (yields ~754 Hz). Why B is correct: - Matches approximate Doppler shift for source receding, where frequency decreases proportionally to source speed away from observer. Why the others are wrong: - A: 19 m/s away yields ~758 Hz, too high for 750 Hz. - C: 21 m/s towards yields ~852 Hz, frequency increases, not decreases. - D: 19 m/s towards yields ~849 Hz, frequency …