An ideal fluid density $800 kg m^{-3}$, flows smoothly through a bent pipe (as shown in figure) that tapers in cross – sectional area from a to a/2. The pressure difference between the wide and narrow sections of pipe are 4100 Pa. At wider section, the velocity of fluid is  $\frac{\sqrt{x}}{6}m{s}^{-1}$ for x = _____  (Given $g=10 m s^{-2}$)