An airport contains 25 terminals which are two on two connected by tunnels. There is exactly 50 main tunnels which can be traversed in the two directions, the others are with single direction. A group of four terminals is called [i]good[/i] if of each terminal of the four we can arrive to the 3 others by using only the tunnels connecting them. Find the maximum number of good groups.
1. **Understanding the Problem:**
   - We have 25 terminals.
   - There are 50 main tunnels, each of which is bidirectional.
   - The rest of the tunnels are unidirectional.
   - A group of four terminals is called *good* if each terminal in the group can reach the other three using only the tunnels connecting them.

2. **Graph Representation:**
   - Represent the terminals as vertices in a graph.
   - Represent the tunnels as edges in the graph.
   - The problem can be translated into finding the maximum number of complete subgraphs \( K_4 \) (complete graphs with 4 vertices) in a graph with 25 vertices and 50 bidirectional edges.

3. **Using Combinatorial Graph Theory:**
   - A complete subgraph \( K_4 \) means that every pair of vertices in the subgraph is connected by an edge.
   - We need to count the number of such subgraphs in the given graph.

4. **Applying Turán's Theorem:**
   - Turán's theorem provides a way to find the maximum number of edges in a graph that does not contain a complete subgraph \( K_r \).
   - For \( r = 4 \), the theorem states that the maximum number of edges in a graph with \( n \) vertices that does not contain \( K_4 \) is given by:
     \[
     \left(1 - \frac{1}{r-1}\right) \frac{n^2}{2}
     \]
   - For \( n = 25 \) and \( r = 4 \):
     \[
     \left(1 - \frac{1}{3}\right) \frac{25^2}{2} = \left(\frac{2}{3}\right) \frac{625}{2} = \frac{1250}{6} \approx 208.33
     \]
   - Since we have only 50 bidirectional edges, which is much less than 208.33, the graph is far from being dense enough to apply Turán's theorem directly.

5. **Counting \( K_4 \) Subgraphs:**
   - We need to count the number of \( K_4 \) subgraphs directly.
   - Each \( K_4 \) subgraph has \(\binom{4}{2} = 6\) edges.
   - Let \( x \) be the number of \( K_4 \) subgraphs.
   - Each edge can be part of multiple \( K_4 \) subgraphs, but each \( K_4 \) subgraph contributes 6 edges.

6. **Using the Edge Count:**
   - Since we have 50 bidirectional edges, we can set up the equation:
     \[
     6x \leq 50
     \]
   - Solving for \( x \):
     \[
     x \leq \frac{50}{6} \approx 8.33
     \]
   - Since \( x \) must be an integer, the maximum number of \( K_4 \) subgraphs is 8.

Conclusion:
\[
\boxed{8}
\]