JAMB Physics 2021 Paper

Concept:An echo is heard when sound travels from the boy to the cliff and returns back to him.So the total distance travelled by sound is twice the distance between the boy and the cliff.Explanation:Let the distance from the boy to the cliff be $d$.Sound travels a total distance of $2d$ in $t = 0.5\, \mathrm{s}$.Given speed of sound, $v = 340\, \mathrm{m\,s}^{-1}$.Using the formula:$$v = \frac{2d}{t}$$Make $d$ the subject:$$d = \frac{v \cdot t}{2}$$Substitute the values:$$d = \frac{340 \times 0.5}{2}$$$$d = \frac{170}{2} = 85\, \mathrm{m}$$Answer:The boy is $85\, \mathrm{m}$ away from the cliff.Correct option: D. 85 m.