Answer
$6.72\times 10^{-10}\ s$
Work Step by Step
Given;
the speed of an electron $v=0.992\ c$
distance moved by electron $d=20\ cm = 20\times 10^{-2}\ m$
Thus;
time taken by the electron to travel a distance d is:
$t=\frac{d}{s}$
$t=\frac{20\times 10^{-2}\ m}{0.992\ (3\times10^{8}\ m/s)}$
$t=6.72\times 10^{-10}\ s$
