From each vertex, N-4 diagonals can be drawn, where N is the number of vertices. The total number of diagonals in a prism will be N(N-4)/2, where N is the number of vertices.
For N=10 (a pentagonal prism), the number of diagonals is (10)(6)/2 = 30.
Half of them are space diagonals.
For N=10 (a pentagonal prism), the number of diagonals is (10)(6)/2 = 30.
Half of them are space diagonals.