A girl standing some distance from the cliff claps her hands and hears the echo 0.8s later. calculate her…

Analyze the question: An echo is sound that travels from the girl to the cliff and then back to her. The 0.8 s is the time for the round trip, so the sound covers twice her distance from the cliff.

Concept: Speed = distance ÷ time, so the total distance travelled = speed × time. For an echo: 2d = v × t, so d = vt / 2

Solution:

• v = 320 m/s
• t = 0.8 s

Total distance travelled by the sound: 2d = 320 × 0.8 = 256 m

Distance from the cliff: d = 256 ÷ 2 = 128 m

Answer: B (128 m)

Common mistakes:

• Using d = 320 × 0.8 = 256 m, forgetting to halve it. That value isn't in the options, which hints you missed a step.
• Always check whether the time given is for a one-way or round trip.

Tip: For any echo question, remember "echo = there and back", so use d = vt/2. If the time is given as the time to hear the echo, always divide by 2.