$T_{t}=\frac{Packet \:size}{Bandwidth}=\frac{1024*8}{256*10^{6}}=32\mu sec$
$RTT=200\mu sec........\left \{ Given \right \}$
So efficiency of sender
$\eta =\frac{1}{1+a}\approx 0.1379\:or\: 13.79........\left \{ where \:a=RTT/T_{t}=200/32=7.25 \right \}$