$\eta = \frac{N}{1+2a}$ , where $N =window$ $size$ , $a = \frac{T_{p}}{T_{x}}$ , $\eta = efficiency$
and also , $\eta = \frac{Throughput}{Bandwidth}$
so , $\frac{Throughput}{Bandwidth}$ = $\frac{N}{1+2a}$
$\frac{1 }{5} = \frac{N}{1+2*5}$
$N = \frac{11}{5} = 2.2 \approx 3$