From A conducting sphere of radius R a cavity of radius R/2 is cut out as shown in the figure. At the centre of cavity a point charge q is placed then, the potential and field at point P as shown in the figure is

Charge -q will be induced on the inner surface of the cavity with uniform surface charge density . Also +q charge will be induced on the outer surface of the metallic sphere with uniform surface density  . Effect produced by -q and +q at the centre of cavity at point P will cancel outTherefore at point P , E = q4πε0d2  and V = q4πε0d ..