Let $S_i, 1\le i\le 10$ be the set of servers and $W_j, 1\le j\le 15$ be the set of workstations.
Connect $W_k$ to $S_k$ for $1\le k\le 10.$ Then connect $W_k$ for $11\le k\le 15$ to $S_i,1\le i\le 10.$
Clearly, one requires 60 cables. Assume that there are fewer than 60 connections between workstations and servers.
Hence some servers would be connected to at most $\lfloor \frac{59}{10}\rfloor =5$ workstations.
Thus, the remaining 9 servers are not enough to allow the other 10 workstations to simultaneously access different servers.