$\left(\mathrm{P}_{1}-\mathrm{P}_{2}\right)=\frac{1}{2} \rho \mathrm{v}_{2}^{2}$
since tap is closed, $\mathrm{v}_{1}=0$
$\left(3.5 \times 10^{5}-3 \times 10^{5}\right)=\frac{1}{2} \times 10^{3} \times v^{2}$
$\mathrm{v}=10 \mathrm{m} / \mathrm{s}$

(Given density of air $=1.2 \mathrm{~kg} \mathrm{~m}^{-3}$ )

