Question
A man stands from a cliff and claps. He hears the echo later. What is the speed of sound in air? Solve in under seconds.
Solution — Step by Step
Echo problems are always round-trip. The sound travels from man to cliff and back, so total distance is . We don’t write any wave equations.
Final answer: .
Why This Works
Echo questions are testing one idea: the round-trip factor of . Once you spot it, the algebra is one line. CBSE Class 9 boards repeat this pattern almost every year — only the numbers change.
For NEET, the same trick applies to SONAR (underwater echoes) and ultrasonic distance meters. Recognise the pattern, multiply by , divide.
Alternative Method
If the question gives speed and time, find distance: . The factor of is the only thing students forget — make it muscle memory.
For NEET-style questions involving two cliffs (echo from each side), use if echoes arrive simultaneously. This appeared in NEET 2021.
Common Mistake
Forgetting the round-trip and writing . This is the most common boards mistake — half the marks gone. Always draw a small arrow showing forward and return paths before computing.