Two neutral conducting spheres of diameters $8 \mathrm{~cm}$ and $2 \mathrm{~cm}$ separated with a distance of $15 \mathrm{~cm}$ between their centres are joined by a thin conducting wire. A charge of $100 \mathrm{nC}$ is given to one of the spheres and the system is allowed to reach electrostatic equilibrium. The electric potential at a point on the line joining the centres of the two spheres where the net electric field becomes zero is ____ V. (Neglect the charge acquired by the wire and $\frac{1}{4 \pi \varepsilon_0}=9 \times 10^9 \mathrm{Nm}^2 \mathrm{C}^{-2}$)