For TCP to work correctly, Packet Life Time $\leq$ Wrap Around Time.
Wrap Around Time is the time in which all the sequence numbers of the Seq. No. field are used, i.e., here when $2^8-1$ bytes of data is transmitted. This should be done so that different packets with same sequence number should not be wrongly accepted by receiver.
So, $40 sec \leq \frac{255 B}{ BW Bps}$ $\rightarrow$ $BW \leq \frac{255}{40}$ $\rightarrow$ $BW \leq 6.375 Bps$