WAEC Physics 2015 Paper

Concept:An echo is heard when sound travels from the girl to the cliff and then reflects back to her.So the total distance travelled by sound is twice her distance from the cliff.Explanation:Speed of sound in air, $v = 320\ \mathrm{m\,s}^{-1}$.Time taken for the echo to be heard, $t = 0.8\ \mathrm{s}$.Total distance travelled by sound = $v \times t = 320 \times 0.8 = 256\ \mathrm{m}$.This total distance covers the journey to the cliff and back.Therefore, distance of the girl from the cliff = $\frac{256}{2} = 128\ \mathrm{m}$.Answer:The girl is $128\ \mathrm{m}$ from the cliff.Correct option: B. $128\ \mathrm{m}$.