\(\frac{{{P^2}}}{{2m}} = e({V_B} - {V_A}) = e(20)\,\,\, \Rightarrow \,\,P = \sqrt {2me(20)} \)
\( \Rightarrow \,\,\lambda = \frac{{6.6 \times {{10}^{ - 34}}}}{{\sqrt {2 \times 9.1 \times {{10}^{ - 31}} \times 1.6 \times {{10}^{ - 19}} \times 20} }}\)
\( = 2.75 \times {10^{ - 30}}m = 2.75\,{\text{Å}}\)
$\left({h}=6.63 \times 10^{-34}\, {J} \cdots\right.$, $\left.{c}=3 \times 10^{8} \,{ms}^{-1}\right)$