or $\mathrm{v}_{2}=\left(\frac{\mathrm{A}_{1}}{\mathrm{A}_{2}}\right) \mathrm{v}_{1}$     $...(i)$

from bernoulli's equation for horizontal flow,

$\mathrm{P}_{1}+\frac{1}{2} \rho \mathrm{v}_{1}^{2}=\mathrm{P}_{2}+\frac{1}{2} \rho \mathrm{v}_{2}^{2}$

$\mathrm{P}_{1}+\frac{1}{2} \rho \mathrm{v}_{1}^{2}=\mathrm{P}_{2}+\frac{1}{2} \rho\left[\frac{\mathrm{A}_{1}^{2}}{\mathrm{A}_{2}^{2}}\right] \mathrm{v}_{1}^{2}$

or $\mathrm{P}_{1}-\mathrm{P}_{2}=\frac{1}{2} \rho \mathrm{v}_{1}^{2}\left[\frac{\mathrm{A}_{1}^{2}}{\mathrm{A}_{2}^{2}}-1\right]$

but $\mathrm{P}_{1}-\mathrm{P}_{2}=600 \mathrm{N} / \mathrm{m}^{2}$

$600=\frac{1}{2} \rho v_{1}^{2}\left[\frac{A_{1}^{2}}{A_{2}^{2}}-1\right]$

$\mathrm{v}_{1}=\sqrt{\frac{1200}{\rho}}\left[\frac{\mathrm{A}_{1}^{2}}{\mathrm{A}_{2}^{2}}-1\right]^{-1 / 2}$

$\mathrm{Q}=\mathrm{A}_{1} \mathrm{v}_{1}=\mathrm{A}_{2} \mathrm{v}_{2}$

$=\frac{6}{\sqrt{10}} \times 10^{-3} \mathrm{m}^{3} / \mathrm{s}$