Purging is nothing but removing unusable data from a system. (Not in the syllabus)
Coming to the question
At t0 both A and B start transmission-
At t1 there will be a collision, which will be exactly at the middle of the link = 20/2 = 10 micros.
at t2 both the station will know about the collision = 10 + 10 = 20 micros.
at time t3 A will be immediately allowed to retransmit cause K=0, but A will wait for one propagation delay time to make sure there is no bit present in the link (purging concept)
So, at time t3= 20+20 = 40 micros, A starts packet retransmission.
Hope it helps.