Ans.

If L be the distance between the centre of suspension and centre of oscillation of the compound pendulum then the time period of oscillation of the compound pendulum is

$\displaystyle{T=2\pi\sqrt{\frac{L}{g}}}$

where, $g$ is the acceleration due to gravity.

For a second pendulum the time period of oscillation is 2 sec.

therefore, $\displaystyle{2=2\pi\sqrt{\frac{L}{g}}}$

or, $\displaystyle{L=\frac{g}{{\pi}^2}}$

or, $L=\frac{980}{3.14}$ cm

or, $L=99.19$ cm