A thin spherical conducting shell of radius $R$ has a charge $q$. Another charge $Q$ is placed at the centre of the shell. The electrostatic potential at a point $P$ at a distance $R/2$ from the centre of the shell is:

The potential at point $P$ is the sum of potentials due to the central charge $Q$ and the charge $q$ on the shell.Potential due to charge $Q$ at distance $r=R/2$:
$$V_Q=\frac{1}{4\pi {ϵ}_0}\frac{Q}{R/2}=\frac{2Q}{4\pi {ϵ}_{0}R}$$
Potential due to the conducting shell with charge $q$: Since $P$ is inside the shell ($R/2<R$), the potential is uniform and equal to the potential on its surface:
$$V_q=\frac{1}{4\pi {ϵ}_0}\frac{q}{R}$$
Total potential $V_P=V_Q+V_q=\frac{2Q}{4\pi {ϵ}_{0}R}+\frac{q}{4\pi {ϵ}_{0}R}$.