An oil pipe line $\backslash ($ A $\backslash )$ is connected to a pressurized air tank through a multi$-$fluid manometer as shown in the figure. If the air pressure in the tank is $50$ kPa $,$ pressure in the oil pipe is $40$ kPa $,$ and the angle of the manometer $\backslash (\backslash$operatorname$\{$limb$\}\backslash )$ is $\backslash (\backslash$theta$=30^\{\backslash$circ$\}\backslash ),$ Calculate the distance $\backslash ($ d $\backslash )$ on the inclined limb of the mercury level difference in the manometer. $($Take the specific gravities as $13\mathrm{.}6$ for mercury and $0\mathrm{.}9$ for oil$).$