Answer
The pendulum's energy has decreased by a factor of 400 in that time.
Work Step by Step
We can write an expression for the initial value of the energy:
$E_1 = \frac{1}{2}m~A_1^2~\omega^2$
Note that $A_2 = \frac{A_1}{20.0}$. We can find the new value of the energy:
$E_2 = \frac{1}{2}m~A_2^2~\omega^2$
$E_2 = \frac{1}{2}m~(\frac{A_1}{20.0})^2~\omega^2$
$E_2 = \frac{1}{400}\times \frac{1}{2}m~A_1^2~\omega^2$
$E_2 = \frac{1}{400}\times E_1$
The pendulum's energy has decreased by a factor of 400 in that time.
