Echo time difference from round-trip distances

Steps:

• Calculate round-trip distance to P: 2 × 50 m = 100 m.
• Calculate round-trip distance to Q: 2 × 200 m = 400 m.
• Find difference in round-trip distances: 400 m - 100 m = 300 m.
• Use time difference of 1.0 s to find speed: v = 300 m / 1.0 s = 300 m/s.

Why B is correct:

• The speed follows from the formula for echo time difference: Δt = 2(d_Q - d_P)/v, rearranged to v = 2(d_Q - d_P)/Δt.

Why the others are wrong:

• A: 250 m/s yields Δt = 300/250 = 1.2 s, not 1.0 s.
• C: 350 m/s yields Δt = 300/350 ≈ 0.86 s, not 1.0 s.
• D: 500 m/s yields Δt = 300/500 = 0.6 s, not 1.0 s.

Final answer: B