1 RTT --> 1 MSS
2 RTT --> 2 MSS
3 RTT --> 4 MSS
4 RTT --> 8 MSS
5 RTT --> 16 MSS
6 RTT --> 32 MSS (64 KB can be sent in 6th RTT , but receiver window is 36 KB, hence we have to limit upto 36 KB)
Hence, in 6th RTT, 36 KB can be sent.
It is asking the time before which the full window can be sent is 5 RTT = 5 * 5 = 25 ms