Time=distance/velocity

=20/300
=0.067secs
s = ut 1/2gt^2
s = 0*0.067 0.5*10*0:067^2
s = 0 5*0.004489
s = 0 0.0224
s = 0.022m

It seems like you have provided a calculation, but I'm not sure what exactly you are trying to calculate. Can you please provide more context or clarify your question?