It is based on the concept of pipelining.See there are five packets.Assume A,B,C,D,E.
A transmits, TT+ journey on link = 50μsec + 200μsec = 250μsec . .................(1)
Now , during this journey of A more packet are introduced on the link.Like when A finished 50 on link,B is introduced on link.Similarly other packets are also introduced.
NOw ,When when A is reached to the end,E has finished landing on link.
So only E 's propagation delay is left out which is 200 ..................(2)
So, overall (1)+(2)=450μsec