For congestion control policy , the condition which is needed is :
congestion window size < receiver window size
which holds here as maximum congestion window size < receiver window size..
Initially window size = 2 KB ( 1 MSS ) i.e. at t = 1 , window size = 2 KB
at t = 2 , window size = 4 KB
at t = 3 , window size = 8 KB
at t = 4 , window size = 16 KB
at t = 5 , window size = 18 KB [threshold reached , after this congestion avoidance]
at t = 6 , window size = 20 KB
at t = 7 , window size = 22 KB
at t = 8 , window size = 24 KB
at t = 9 , window size = 26 KB
During this transmission(means same as after 8 transmissions) , timeout occurs and hence transmission at t = 9 fails..
So applying multiplicative decrease (as timeout means strong possibility of congestion).
So now new value of threshold = Current window size / 2 = 13 KB and slow start phase starts again
Now at t = 10 , window size = 2 KB
at t = 11 , window size = 4 KB
at t = 12 , window size = 8 KB
at t = 13 , window size = 13 KB (threshold value reached)
at t = 14 , window size = 15 KB
at t = 15 , window size = 17 KB
at t = 16 , window size = 19 KB
Hence number of RTTs involved to send data of 18 KB = 7 RTT (as till t = 15 , we can send only 17 KB but we need to send 18 KB so we need one more RTT and timeout occurs during t = 9 , so we need to see the time needed after timeout which is at t = 9 , so we need to consider time interval between t = 9 to t = 16).
Hence time taken = 7 RTT = 7 * 10
= 70 ms
