We know that, when TimeOut occurs, Threshold set to $\dfrac{cwnd}{2}$

Here, Threshold will be set to $\dfrac{64}{2} = 32 \: KB$ after TimeOut.

& Maximum Segment Size (MSS) is $4 \: KB$.

So, after TimeOut cwnd is set to $4 \: KB$.

at $t = 1, cwnd = 4 \\ t=2, cwnd = 4+4 = 8 \: KB \\ t = 3, cwnd = 8+8 = 16 \: KB \\ t = 4, cwnd = 16+16 = 32 \: KB \quad \color{Red}{\text{ [Threshold reached]}} \\ t = 5, cwnd = 32+4 = 36 \: KB \quad \color{Blue}{ \text{[Entering into Congestion Avoidance phase]}}\\ t = 6, cwnd = 36+4 = 40 \: KB \\ t = 7, cwnd= 40+4 = 44 \: KB \\ t = 8, cwnd = 44+4 = 48 \: KB \\ t = 9, cwnd = 48+4 = 52 \: KB \\ t = 10, cwnd = 52+4 = 56\: KB \\ t = 11, cwnd = 56+4 = 60 \: KB\\ t = 12, cwnd = 60+4 = 64 \: KB$

$\therefore 4 \mid 8 \mid 16 \mid 32 \mid 36 \mid 40 \mid 44 \mid 48 \mid 52 \mid 56 \mid 60 \mid 64 \\ i.e \text{ it is taking 11 RTT's to get back to 64 KB congestion window}$

Now, the TCP connection completed $11 \: RTT's$ in just $7.2 \: sec$ i.e $7.2 \times 10^3 \: ms = 7200 \: ms$

One RTT will take $ \left ( \dfrac{7200}{11} \right ) \: ms = 654.54 \: ms$