Threshold = 40 / 2 = 20 KB
First transmission = 2 KB
Second transmission = 4 KB
Third transmission = 8 KB
Fourth transmission = 16 KB
Fifth transmission = 20 KB
Sixth transmission= 22 KB
Seventh transmission = 24 KB
Eight transmission = 26 KB
Ninth transmission=28 KB
A total of 9 RTTs are required to reach the congestion window of 30 KB.
9 RTTs = 9 * 50 = 450 msec