A hollow spherical conductor of radius 4 cm carries a positive charge of $\frac{10}{9}\times {10}^{-10}\mathrm{C}.$ At what distance (in cm) from centre of sphere should a positive point charge of $\frac{40}{9}\times {10}^{-10}\mathrm{C}$ be placed so that the potential of the conductor becomes 75 V ? Assume potential to be zero at infinity.