Answer
$8\times 10^7 \;\rm m/s$
Work Step by Step
Using the conservation of energy principle and assuming that there are no external forces exerted on this system [the electron + the charged sphere].
$$K_i+U_{ie}=K_f+U_{fe}$$
The minimum speed needed for the electron to escape is the speed that makes the electron reach an infinity distance and that $v_f=0$ m/s
$$K_i+U_{ie}=0+0$$
$$\frac{1}{2} m_ev_{\rm esc}^2+(-e)V_i= 0$$
where $V$ for a charged sphere on its surface is given by $V=kq/R$
$$\frac{1}{2} m_ev_{\rm esc}^2=\dfrac{keq}{R}$$
$$ v_{\rm esc} =\sqrt{\dfrac{2keq}{m_e R}}$$
Plug the known;
$$ v_{\rm esc} =\sqrt{\dfrac{2(9\times 10^9)(1.6\times 10^{-19})(10\times 10^{-9})}{(9.11\times 10^{-31})(0.5\times 10^{-2})}}$$
$$ v_{\rm esc} =\color{red}{\bf7.95\times 10^7}\;\rm m/s$$
