2

A pendulum is a piece of string attached to a 20 g mass that if you double the length it will take twice as long to swing.

decrease by a factor of 4

The formula for the frequency of the pendulum is w2=g/l

if you wish to double your period w1, you want to have w2 = 2*w1

The needed length of the pendulum is then l2 = g / w22 = g /(4 * w12) = 0.25 * g / w12 = 0.25 * l1

l2 / l1 = 1/4

You must shorten the length of the pendulum to 1/4 of its former size.

Compound pendulum is a physical pendulum whereas a simple pendulum is ideal pendulum. The difference is that in simple pendulum centre of mass and centre of oscillation are at the same distance.