Correct Answer - `40`
Electric field on particle `E=E=(-DeltaV)/(Deltax)=E=(-[250-(-250)])/(20xx10^(-2))=-2500 V//m`
Acceleration of charge particle `=(qE)/m=(1.6xx10^(-19)xx2500)/(16xx10^-31))=2500xx10^(11)m//s^(2)`
thus time taken `= sqrt((2xx5)/a) {S=1/2 at^(2))`
`=sqrt((2xx20xx10^(-2))/(2500xx10^(11))=4xx10^(-8) sec = 40 nS`