Vietnam University entrance Exam problem (Don't be fooled)

A light springs have stiffness coefficient $K=20\text{ N/m}$ hanging on to a fixed point, the lower of a string attaching small object $A$ weighing $100\text{ g}$ ; $A$ is connected to a small object $B$ weighing $100\text{ g}$ with a soft rope, piece, lightweight, non-stretch and long enough. From the equilibrium position of the system, pull down $B$ vertically to $20 \text{ cm}$ and then release with zero initial velocity. When object B began to reverse its movement, it suddenly slip from rope. Ignore any obstacles, take $g = 10\text{ m/s}^2$ . The time between when the object $B$ were come out off the rope until fall to the initial position when $B$ was released is?