Given Bandwidth = 20 kbps, Tp = 300 ms, Frame Size = 50 Bytes = 50 * 8 bits,
N = 10
Now, Tt = $\frac{Frame Size}{Bandwidth}$ = $\frac{50 * 8 b}{20 * 10^{3} bps}$ = 20 ms
We know Throughput = Efficiency * Bandwidth = $\frac{N}{1+2*\frac{Tp}{Tt}}* Bandwidth$
=$\frac{10}{1+2*\frac{300}{20}} * 20 kbps$ = 6.45 kbps