since no force is acting on the small block, hence it will have no acceleration w.r.t. earth. now,

F=10 N

mass of block A=5kg

acc. = 2m/s^2

distance moved to separate from the small block=20cm=0.2m

initial velocity = 0m/s

s = ut + at^2/2

0.2 = 2.t^2/2

t^2 = 0.2

t = 0.45s (approx)

do rate