Given.
$Bw = \ 100 \text{ Kbps}$
Let the total number of stations be $N$
Throughput of station $= \frac{\text{Total Data Transfer}}{\text{Time taken}} = \frac{1000 \ \times \ 8 \ bits}{50 \ sec}$
$\eta_{\text{pure aloha}} = 0.184$ (max)
Throughput of link $ = \eta_{\text{pure aloha}} \times Bw = 0.184 \ \times \ 100 \ \text{ Kbps} = 18.4 \text{ Kbps}$
$\therefore $ Throughput of link $=$ Throughput of station $\times$ Total number of station
$18.4 \text{ Kbps} = \frac{1000 \ \times \ 8 \ bits}{50 \ sec} \times N$
$N = \frac{18.4 \times 10^3 \times 50}{1000 \times 8} = 115$