Initial amplitude, Ai=A0 Final amplitude Af=0.9A0 Case 1 Time t1=5s Case 2 Time t2=20s Let, amplitude after 20s be A′. As we know that, A=A0e−α ∴0.9A0=A0e−5α ⇒e−5a=0.9 Again, A′=A0e−200 Now on powering Eq. (i) by 4 , we get ⇒(e−5α)4=(0.9)4 ⇒e−20α=(0.9)4 ∴ From Eq. (ii), A′=A0e−200=A0(0.9)4=0.6A0≈0.59A0