Since both sound waves travel the same round-trip distance, speed is inversely proportional to time. Taking the ratio of time taken gives the speed ratio of air to water as 1:4.5 or 2:9.
Class 9 Science Exploration Chapter 10 Important Questions from Sound Waves
SQP from Class 9 Science Chapter 10 Sound Waves Characteristics and Applications
Both sound waves cover the identical distance to the vertical cliff and back. Because wave speed equals distance divided by time, speed is inversely proportional to the transit time. Since the time taken in air is 4.5 times that in water, the ratio of the speed of sound in air to that in water is 1 to 4.5, which simplifies to 2:9 (or roughly 0.22).
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