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Q.
The amplitude of a damped oscillator decreases to $0.9$ times its original magnitude is $5\, s$. In another $10\, s$ it will decrease to
a times its original magnitude, where $\alpha$ equals
Amplitude decreases exponentially. In 5 s it remains
0.9 times. Therefore in total 15 s it will remains (0.9) (0.9)
Answer is 4. (0.9) = 0.729 times its original value.