Answer
$\lambda = 6.21\times 10^{-11}~m$
Work Step by Step
We can find the minimum wavelength:
$q~\Delta V = \frac{hc}{\lambda}$
$\lambda = \frac{hc}{q~\Delta V}$
$\lambda = \frac{(6.626\times 10^{-34}~J~s)(3.0\times 10^8~m/s)}{(1.6\times 10^{-19}~C)(20.0\times 10^3~V)}$
$\lambda = 6.21\times 10^{-11}~m$
