JAMB Physics 2020 Paper

Concept:Echo time is the time for sound to travel to the cliff and back.A decrease in echo time means the ship is closer to the cliff.Explanation:Initial echo time $= 3.5\,s$.Later echo time $= 2.5\,s$.Difference in echo time $= 3.5 - 2.5 = 1.0\,s$.This $1.0\,s$ is the total extra time for sound to travel to the cliff and back.So one-way time difference $= \frac{1.0}{2} = 0.5\,s$.Distance moved closer $= \text{speed} \times \text{one-way time difference}$.$= 250 \times 0.5 = 125\,m$.Answer:The ship is $125\,m$ closer to the cliff. Option D.