Another Brainstroming ques

A plank of mass 2 kg and length 1m is placed on a horizontal floor. A small blockk of mass 1 kg is placed on the top of the plank, at it's right extreme end. The coeff of friction b/w the plank and the floor is 0.5 and that b/w plank and the block is 0.2. If a horizontal force = 30 N starts ating on the plank to the right, show that the time after which the block will fall off the palnk is 2/3 sec.