Calculate the pressure difference between vessels A and B when the unit weight of water is $9\mathrm{.}8$kN$/$m$3,$ the unit weight of oil is $8$kN$/$m$3,$ and the unit weight of mercury is $133\mathrm{.}4$kN$/$m$3,$ y$1=300$mm$,$ y$2=500$mm$,$ y$3=400$mm$,$ and y$4=200$mm$.$