Answer
${\Delta}K.E=20.0$ keV
Work Step by Step
It is given that n=100.
Therefore, its KE as it completes 100 passes through the gap is:
${\Delta}K.E=n(200eV)$
${\Delta}K.E=100(200)$
${\Delta}K.E=20000$
${\Delta}K.E=20\times 10^3$
${\Delta}K.E=20KeV$
