Propogation Delay (TProp) = 10 * 10-3 seconds
Bandwidth (R) = 109 bps
packet size (L) = 1000 * 8 = 8000 bits
Transmit Time (TTrans) = $\frac{L}{R}$ = $\frac{8000}{10^9}$ = 0.008 * 10-3 seconds
$Utilization = \frac{T_{Trans} * L}{T_{Trans} + 2 * T_{Prop}}$
$0.8 < \frac{0.008 * 10^{^{-3}} * L}{ 0.008 * 10^{-3} + 20 * 10^{-3}}$
$L > \frac{20.008 * 0.8}{0.008} = 2000.8$ Packets
So L should be 2001 Packets.