Find the value of $g$ (in $m\;s^{-2}$) up to approximate significant figures stating the uncertainty in the value of $g$, given the following information:

Question

The time period of a simple pendulum is given by $T = 2\pi \sqrt {\large\frac{L}{g}}$. The measured value of $L$ is $20.0 \;cm$ using a scale of least count $1\;mm$. The time $t$ for $100$ oscillations is found to be $90\;s$ using a watch of least count $1\;s$.