A $2 \mathrm{~m} \times 1.5 \mathrm{~m}$ tank of $6 \mathrm{~m}$ height is provided with a $100 \mathrm{~mm}$ diameter orifice at the center of its base. The orifice is plugged and the tank is filled up to $5 \mathrm{~m}$ height. Consider the average value of discharge coefficient as 0.6 and acceleration due to gravity $(g)$ as $10 \mathrm{~m} / \mathrm{s}^{2}$. After unplugging the orifice, the time (in seconds) taken for the water level to drop from $5 \mathrm{~m}$ to $3.5 \mathrm{~m}$ under free discharge condition is $\_\_\_\_\_\_\_\_$ (rounded off to $2$ decimal places).