It’s important to distinguish between the system (network) and the individual station even though numerically both of them are having the same efficiency which is 50%. Consider that each station gets a time slot of $t_t + t_p$ then the efficiency in this case will be $\frac{t_t }{(t_t+ t_p)}$. Likewise each individual station will have the same efficiency. Then the total efficiency of the system(network) consisting of n stations can be written as
$\frac{nt_t}{n(t_t+t_p)} = \frac{nt_t}{(nt_t+nt_p)} = \frac{T_t}{T_t+T_p}$
where $T_t$ is the total transmission time by all n stations and $T_p$ is the total propagation time of all n stations
The most important thing to note here is that $T_t = nt_t$ and $T_p = nt_p$.
The transmission time and propagation time given in the question is the total sum of individual stations. Likewise, the bandwidth is also of the system which is equal to the sum of bandwidth of individual stations.
If we are given the effective bandwidth of each station then we can get the number of stations by calculating the effective bandwidth (throughput) of the system and dividing it by the effective bandwidth of each station, since the effective bandwidth of the system is equal to the sum of indiviual effective bandwidth.