tcp congestion avoidance
When we are in AC mode every rtt(round trip time) the congestion window increase by 1(MSS).
Here our current congestion window(cw) is 60 and we know that during transmission round t we get 3 duplicate acks-so our window is now 30MSS,now we also know that from t to t+1 for instance our window increase by 1,so from t to t+10 our window increases by 10-which means our window is now 40.
We know that during the t+10 round we will get 3 duplicate acks,so now our window will become 40/2=20.
From round t+10 to t+20 our cw increase by 10 more-so now its 30,and in addition we know we will get 3 duplicate acks-which means now our window will be 30/2=15MSS