JAMB Physics 2018 Paper

Concept:An echo occurs when sound travels to a reflecting surface and returns to the listener.Hence, the total distance travelled by the sound is twice the distance between the boy and the hill.Explanation:Let $d$ be the distance from the boy to the hill.The sound wave moves from the boy to the hill and then back to the boy.So, total distance covered by sound $= 2d$.Using the relation:$$\text{Total distance} = \text{speed} \times \text{time}$$Substitute the given values:$$2d = 340 \, \text{ms}^{-1} \times 0.8 \, \text{s}$$$$2d = 272 \, \text{m}$$Divide both sides by $2$:$$d = \frac{272}{2}$$$$d = 136 \, \text{m}$$Therefore, the boy is $136 \, \text{m}$ from the hill.Answer:The correct option is B: $136 \, \text{m}$.